Williams is good but I think he has some very serious flaws in his game. He blocks shots but he is not great defensively. His post defense is atrocious and his recognition on when, where and who to switch to needs to get better. Offensively at this point he's either a dunk or an occasional jump hook or baby baseline jumper. The guy has got a lot of potential but has a lot of work to do. Good highlight reel guy though.
Out of the guys I've had a chance to see play Yi looks like the guy with the most potential. He has infinite range on his shot, can score in and out of the post, as well as being an excellent passer, good rebounder and dribbler of the ball. A lot of people compare him to Dirk but I think he's going to be better then him. Look for him to make a big jump in his 2nd season a lot like Rudy Gay has this season. With all the hype Durant and Oden got it would be really funny if Yi turned out to be the best player from the '07 draft.