Smith in my opinion is the better talent and is much more proven.
That said, Carr and Vinny have no business being starting QBs at this point. Carr holds the ball waaaaaaaaaaaaaaay too long, and Vinny is servicable for a half but then gets hurt. Smith has put up horrid numbers since Delhomme got hurt, and there is no reason to think anything will change.
Roy - while he hasnt had the most outstanding season so far, he is on a team that is playing well, and has finally balanced out its offense. They arent throwing 50 times a game like they were at the begining of the season, but that is actually a good thing. It means less hits on Kitna, and it makes the D respect the run and not just drop everybody back. Roy still is the best talent on DET at WR, and its only a matter of time until he starts to have a bigger impact.
Roy would be the way to go for me