Hibbert was a senior, his game was far more developed on both ends of the floor. Melo can not put the ball in the basket to save his life. He may be a better athlete than Hibbert but that does not matter as much at the 5 position. I frankly do not see the need in getting another version of Perkins. That style of play is being phased out in today's NBA.
Id pick him at 21. While he's not an offensive threat, nor I think will learn how to be, that athleticism and big frame has potential to be a great interior defender. While there are not a lot of two way centers (offense and defense), having an imposing 7 ft body in the middle can be a difference maker.
He does have a lot to learn but not everyday you can find a 7 ft guy on pick 21 who can jump out of the building and, with some coaching and proper development, could be a defensive beast.