CP nailed it. This topic has been discussed to death.
I guess it won't hurt to beat the dead horse a little bit more though.
These two backs are my top two rookies for the 2008 class. I could see either one of them becoming the better pro player, but I favor Mendenhall because he seems like a safer pick. I think he'll be more durable and I love the fact that he went to the Steelers. A lot of people were down on him because of his situation. I actually think Pittsburgh was one of the 3-4 best places in the entire NFL he possibly could've gone. They have a great track record of developing their first round picks and they have a talented young offensive infrastructure (Roethlisberger, Holmes, Miller) that should help Mendenhall in the same way that the Indianapolis supporting cast has helped Addai. Unless he's a total scrub, Mendenhall is probably going to put up pretty good numbers in Pittsburgh's system. The only thing that worries me about him is his lack of top overall athleticism. He's not as explosive as the elite NFL starters and he may end up being more of a pounder than a big play type.
Stewart is the better athlete on paper. He's a freak of nature from a size/speed/combine standpoint. He runs in the 4.4 range at 235 pounds, bench presses a small house, has a monster vertical leap and broad jump, and closely resembles The Thing from Fantastic Four. At his best, he looks unstoppable. He was a man among boys in his bowl game. His ceiling is very high. That said, there are some issues with him. He's so big that he sometimes appears to have trouble regaining his momentum and making smooth cuts. He takes too many hard hits and has had trouble staying healthy. His body language sometimes has a lackadaisacal quality, almost like he doesn't want to be out there. Character issues are minimal though and he demonstrated the ability to play through pain last season.
Overall, I'm pretty high on both of these guys. They're both high quality first round prospects with the potential to develop into true every down backs. I won't be surprised if both of them are consensus top 10 dynasty RBs by 2010. IMO they become viable picks in startup dynasty drafts at about the RB13-RB18 range.