It's hard to say. Carter's got a great team behind him. I don't think I can say the same about Philadelphia. Philly's entire offensive strategy revolves around getting the ball to Iverson and seeing if Iverson can shoot the ball or take it to the hoop. If not he passes off to a teamplayer who will initiate another iteration of trying to get the ball to Iverson to see if he can shoot the ball or take it to the hoop. In desperation they'll get the ball to McKie and they're lucky he is a clutch player despite him being underutilized.
Now you could say Iverson is the better player because he has to carry the weight of the team on his shoulders. But on the other hand, it gets pretty easy to get scoring opportunities when you're the only man on the team who can capitalized on those opportunities. So he gets more opportunities to score than Vince does.
On the other hand, with Carter having such a great team behind him, if the going gets tough for Carter, the likes of Antonio Davis, Alvin Williams, Chris Childs or Dell Curry can pick up the slack and they do. He's also got some great defenders and clutch players that make offensive opportunities a little easier to come by.
I don't know enough about Iverson's defensive abilities to comment but I have to say that Carter has got to work on his defense - he's getting better though. Additionally, Carter has got to learn how to get away from the double team.
ditto the previous comment. however, i'd like to add that iverson is by far the most dominant ~6' player ever. nothing really special about carter, besides his lack of defense. put him in the same game as kobe, and he'd be owned