I wouldn't dismiss Sullinger just because he isn't athletic. He has great post moves and footwork, and is a very good rebounder despite his lack of athleticism. Think of other forwards who can score the ball without athleticism - Luis Scola (AKA "Best player below the rim")and Zach Randolph come to mind. The quality of his defense remains to be seen, but I think Sullinger was a great pick.
As for Melo, he completely changed his physique in one year and improved drastically over that period of time. He will no doubt be a project, but the potential clearly is there. He's only been playing for 6 years, and he's going to learn a lot of KG (who I think is coming back).