One's an absolute creep… the other? Dunno

Green Lantern/Warrior (Guy Gardner)- Guardsman (Michael O’Brien)

I cannot tell how Guy Gardner was initially characterized when he first appeared in the Green Lantern books in the late ‘60s and 1970s. But Keith Giffen and J.M DeMatteis decided that he would come forth as the DCU’s biggest, non-villainous asshole when they took over the Justice League franchise in 1987. And that has been the way Gardner’s been played since, often humorously, which suits me fine.

The Guardsman? Don’t remember a goddamn thing other than he worked closely with Tony Stark after antagonizing him, wore an armor similar to Iron Man's and had red hair. All of which will do the trick for me.

Commonalities:
Both are alternates and antagonists for the main character in their respective franchises, with identical M.O.s.

Differences:
I don’t remember anything about whether the Guardsman was a jerk or not.

Alternate histories:

GG/GL: Policeman Guy Gardner is distressed when his insane brother perishes in battle with Green Lantern (Hal Jordan); he incorrectly believes that Jordan killed the elder Gardner. He appropriates the same power ring his brother used and attacks Jordan. But Gardner reconciles with Jordan, and the latter hires the former to provide security at Ferris Aircraft. Gardner masters the alternate power ring, and eventually becomes head of security at S.T.A.R. Labs.

GU/MO: Michael O’Brien is an aggressive young man who, while practicing social work, is nearly picked by the Watchers to become their representative on Earth. Instead, the Watchers select Tony Stark, and O’Brien becomes their alternate representative, despite his resentment towards Stark and virtually everyone he encounters. When Stark is displaced, O’Brien becomes the Guardsman, the only individual on Earth to wear the Nova Corps armor, and joins the Avengers. His sociopathic behavior increases, and he is disliked by most of his teammates intensely. The Guardsman has continued his association with the Nova Corps since Stark’s return.

When black characters replace white ones, look out!

Green Lantern (John Stewart)- War Machine (Jim Rhodes)

At any particular time, you can count on that visitors to the DC comics message boards are complaining about which Green Lantern should be emphasized. In fact, seeing as comic book nerds are pioneers in the field of computer-enabled crying and bitching, the replacement of Kyle Rayner for Hal Jordan as Green Lantern set the nascent ‘Net aflame in the early ‘90s.

These days, there’s a fierce debate re: John Stewart taking Jordan’s spot in Justice League book on the boards. does African-American writer Dwayne McDuffie have a anti-white agenda, these absurd people wonder (http://dcboards.warnerbros.com/web/forum.jspa?forumID=29209048&start=15)? Their outrage is further fueled by the fact that Stewart took Jordan’s place in the animated Justice League Unlimited program, and that the rapper Common was mooted to play Stewart in the now aborted JLA film.

As for Rhodes, the character took Tony Stark’s place as Iron Man when the former succumbed to alcoholism in the early-mid ‘80s, has kicked around since and has been immortalized by Terence Howard in “the best reviewed super hero film ever.

Commonalities:
Both took over for the respective franchise’s primary characters in the 1980s, probably for the sake of diversity. And both have military backgrounds.

Differences:
Only that Stewart is an architect and a space cop, Rhodes isn't; so again, we cheat…

Alternate histories:

GL: John Stewart is a marine who brings Ferris Aircraft chairman Hal Jordan back to America shortly after Jordan escapes from hostile forces via his prototype “power ring.” Stewart is then employed as an architect at Ferris Aircraft and is one of Jordan’s most trusted aides. When Jordan becomes debilitated by alcoholism, Stewart assumes the role of Green Lantern alongside the west coast Justice League expansion team. Stewart then works alongside a recovered Jordan; becomes paranoid when the ring affects his thought processes; recovers and then takes over again when it appears that Jordan has died; and has since worked sporadically with Jordan and other operatives.

WM: Jim Rhodes is chosen by the Watchers as an alternate for Tony Stark, the main representative of the Nova Corps for Earth’s solar system. When Stark leaves the Corps for a time, Rhodes takes over; when Stark loses his mind and then becomes an ethereal spirit of vengeance, Rhodes and Richard Ryder divide his duties. Finally, Rhodes serves with a resurrected Stark as the dual representatives of the Nova Corps and as a member of the current lineup of the Avengers.

Now they become death, destroyer of worlds

Krona- Galactus

Sure, there were cosmic, universe-threatening demi-gods in comic book fiction before 1966’s Fantastic Four no. 66. But the story that spread out into the next three issues, now known as “the Galactus Trilogy,” raised the bar. The very idea of Galactus, a being that consumes the essence of planets, knocked readers on their ear: when you take into account that the tale introduced the Silver Surfer, it’s probably the single greatest achievement of Stan Lee and Jack Kirby’s ‘60s partnership.

I’m not entirely sure, but I believe that Krona, a mad scientist from the race that would evolve into the Guardians of the Universe who obsessively seeks the secret of the creation of the universe (or multiverse), was ignored as a fecund antagonist after the character was introduced in 1965’s Green Lantern no. 40. But he was established with Galactus-level significance at the very least in the early ‘80s and afterwards.

Commonalities:
Both are shown to be present near the creation of the universe and are enmeshed in a process of destruction on a vast, cosmic scale.

Differences:
Krona is typically presented as malevolent, whereas Galactus is essentially a morally neutral force of nature.

Alternate histories:

KR: Krona is the sole survivor of the universe that preceded our own: he emerges after the “big bang” and exists afterwards as a force of nature introducing entropy to one planet at a time. Krona uses heralds, such as J’onn J’onzz, to evacuate these planets; he arrives on Earth to some years ago and is opposed by the Doom Patrol and eventually most superhuman operatives on Earth and throughout the cosmos.

GA: Galan is a renegade scientist native to the planet that would later yield the race of “Watchers”: he becomes obsessed with witnessing the “big bang” that creates the universe. But doing so has been foretold to create a process by which the universe will gradually be eviscerated. Known as Galactus in the untold billions of centuries afterwards, he is opposed first by Iron Man and the Nova Corps and eventually by most superhuman operatives on Earth and throughout the cosmos.

The starting lineup of the West Coast Justice League, 1984

Green Arrow
Black Canary
Green Lantern
Captain Atom
Vixen

Young hepcats/Space Cops

Green Lantern (Kyle Rayner) - Nova

In 1994, while I was barely paying attention, DC brass decided to introduce a new, young hepcat version of Green Lantern: in doing so, they turned Hal Jordan into a crazed bad guy…

And fandom screamed… my understanding is in the 14 years since, Rayner has gained some measure of fan acceptance as one of four Green Lantern corpsmen form Earth.

As for Nova: apparently Marv Wolfman thought that late ‘70s Marvel needed a series with the sense of wonder of early Spider-Man, with perhaps a soupcon of GL. Nova was the result: methinks the character has endured long past any expectations.

Here we see how I get around the fact that Tony Stark was never a space cop: when the shoe’s on the other foot, he’s a member of the Nova Corps who went nutso and was replaced by a younger guy. AKA cheat cheat cheat cheat…

Commonalities:
Both characters were conceived for the identification of the intended audience —young dudes. And both are representatives of an interstellar police force.

Differences:
Nova makes no hard light constructions via a “power ring.”

Alternate histories:

GL: High schooler Kyle Rayner is selected at random by a dying alien representative of the Green Lantern Corps to take his place. Rayner assumes the abilities incumbent to the interstellar police force’s power ring, and encounters many in the metahuman community before serving off world alongside the Corps for a year. Rayner abandons his duties and lives as a normal human for a time before he is pressed back into service with a team of superhumans. He has recently left Earth to take part in a galactic conflagration, in which he become the most powerful Green Lantern.

NO: When Iron Man, Earth’s representative of the Nova Corps, loses his mind and destroys the Corps, Richard Ryder is picked at random to be the last representative of the interstellar peacekeeping force. While Ryder learns on the job, he joins the Avengers. Eventually, Iron Man and the rest of the Nova Corps are revived, and Ryder journeys into space to serve in various galactic conflagrations.

The starting lineup of the Justice League of America, 1963

Superman
Green Lantern
Atom
Insect Queen
Blockbuster

first recruit/replacement for the founding dangerous berserker— Batman

The Capitalist and the Space-Cop

Green Lantern (Hal Jordan)- Iron Man

Big time cheating here…but to me Hal Jordan and Tony Stark are both products of the late ‘50 and early 60s Cold War/New Frontier/ Kennedy diaspora.

Stark has been occasionally written as an arrogant smoothie and a drunk, but always as Marvel’s premiere capitalist: often his early exploits (and certainly his origin) found him combating dirty commies. He’s now the key character in the MU; obviously, this is to set up the summer blockbuster that bears his code-name.

As for Jordan, he was written in the same rationalistic, personality-free manner of all DC characters in the 1960s. Denny O’Neil wrote him as the representation of “law and order” in his celebrated GL/GA run; I’m not entirely sure, but I believe Mark Waid introduced the characterization of Jordan as a cocky Californian.

So, while their specific heroic functions differ, it seems that their earthly preoccupations are hardly dissonant.

Commonalities:
As I said above, Jordan and Stark are confident Alpha dudes. Stark’s armor and gizmos aren’t as versatile as Jordan’s power ring, but both shoot laser beams and have sensory capabilities. And both have "died" and otherwise have seen their status(es) thrown hither and yon in ways tough for me to fathom.

Differences:
Stark is super-rich on a Bruce Wayne level and runs a multinational, whereas Jordan works for an aerospace corporation. Jordan is, essentially, a flyboy and one of millions of space cop answering to an Interstellar authority. Stark has no such “cosmic” orientation, so as you can see below, here be a BIG cheat…

Alternate histories:

GL: Ferris Aircraft chief executive/test pilot Hal Jordan is overseeing his company’s contracts in a war zone overseas, when he is wounded and captured by hostile forces. He’s only able to recover by collaborating with another captive on a hybrid mystical/technologically-derived “power ring” that creates solid light constructions and rays. He returns to the United States and continues his duties running Ferris while posing as Green Lantern, the company’s in-house extranormal operative. After founding the Justice League of America, Jordan struggles with alcoholism (at which time colleague John Stewart assumes his duties as GL), an apparent death and the collapse and then revival of Ferris before ascending to the post of U.S. Secretary of Defense. He recently has led the drive for superhuman registration against opposition from many former allies, and now leads a version of the JLA and has been appointed director of government agency Checkmate.

IM: Tony Stark is an inventor for a massive technology/munitions manufacturer/multinational corporation. He is spirited away to encounter an ailing member of the Nova Corps, an interstellar, inter-species police force supervised by the Watchers, an ancient, nearly omnipotent race. The being dies after appointing Stark to the Corps, upon which he is given a uniform he then modifies into an armor equipped with various weapons and devices. As Iron Man, he patrols Earth’s solar system and the Earth itself, founds the Avengers and works with other earthlings like Iron Men/Nova Corps representatives James Rhodes and Michael O’Brien. For a time, he travels around America with fellow Avenger Hawkeye, takes on a number of jobs and briefly renounces his Iron Man identity. The Nova Corps is eventually disbanded, and Stark loses his mind and goes rogue for a time: he is replaced by one Richard Rider. His spiritual essence becomes wedded to an omnipotent force before he returns to the earthly plane as Iron Man once more.