I have a 5* of him so I'm biased lol
OG Iron Man is useable and groot is probably far worse then him so I’d say that he should probably receive the buff first. Also Iron Man is meant to be a starting champion, he’s meant to be basic and simple and because of this I wouldn’t mind if he stayed that way for years at least it wouldn’t put off new players who just come to play for fun. Also there is already another version of Iron Man who is genuinely good in a lot of places so if you’re one of those “I want to play with my favorite heroes from the comics” players then an option is available. Think about the poor GOTG fans who have Drax, Gamora, SL, Groot and Rocket to choose from. (I know these champions are useful in their own ways but when you get a champion you want something great, not something borderline good most of the time)Because of these reasons I really do believe that Groot should be buffed first.
I have a 5* of him so I'm biased lol nice username @FunkoPop