I have more confidence in McLemore than Williams.
Williams is too much the tweener - not a good enough creator, not flexible enough, smooth enough, or quick enough to guard a lot of 3s, but at the same time, not enough of a power player for the 4. Also, his flat-footedness is fairly commonplace. He doesn't have very good anticipation, or his head isn't into the game, or he's just not competitive enough. Maybe a mixture of all of the above. He's probably not going to start in the NBA, so he needs to make an impact on the defensive end, which he hasn't done for the most part.
McLemore may not be all that great either, but this is his first season, so there's no way I write him off. He has too much athletic ability. At this point, I just see a player who is very, very young. He probably needed to stay at Kansas for two more years. We might have to wait for his third season to see what he really can do. At least he has a great work ethic. Usually, guys with great work ethics and very good athletic ability end up doing well in the NBA.