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.