Williams thinks he's a SF
He has much more deficiency as a SF than he does as a PF. He doesnt handle the ball well enough, his perimeter skills would be considered below average for an NBA SF, he may not be quick enough on his feet to guard top SF, basically he has alot more to overcome as an NBA SF than he does at PF. He will and should be an NBA PF.
I think for him to be his best he is going to have to be a 3 in the NBA. He might have to lose some weight and get a little quicker, but I think he could do it. I think he just might be too small as a starting 4 and I don't think he is much of a rebounder. Sure a guy like Paul Milsapp and DeJuan Blair are smaller at the position, but they are beastly and also really good rebounders. I think no matter what he is going to be able to play a little bit of both. I'm really not worried about him on either end offensively. It's more of a concern about his defense on the perimeter and/or in the post. His athleticism should help him out in either area though.
He can be great at both positions. People are forgetting how much he can train and work on his body to suite either position. Not to mention how much he will develop his ball handling skills, as well as his post game. So he very well could be a small forward, and a damn good one. It all depends on what he and his coaches thinks their need is. If Arizona had a couple of NBA caliber bigmen on their roster, people would probably question his ability to play power forward. I think sometimes when a player is versatile and plays multiple positions it gets mistaken as some kind of a weakness. A player with no position is a different story, but Williams is clearly not that.
His game won't be drastically different either way, he will still play both forward spots. This topic is a non-factor in terms of Derrick's career in the long run.
http://basketball.realgm.com/wiretap/212573/Derrick_Williams_Sees_Himself_As_NBA_Small_Forward