The only thing I would give Rak over Fab is rebounding. The dunk Rak missed yesterday would have been made by Fab. Fab is a better shot blocker both on and off the ball. I do like how Rak stays on his feet more and tries to play more position defense (probably the main reason he's a better rebounder), but against a bigger 5 I'm afraid he'll get pushed around. Offensively Fab was also starting to show some occasional post moves and had been good all year at making himself available for Scoop's lobs.
I'm not a JB apologist, but I'm ok with him playing Rak at the 4 this year. That's his position of the future (especially since I'm sure he has NBA aspirations) and his struggles this year will benefit him next year. Rak played a great game yesterday and I hope he can build off it, but let's not make him out to be what he isn't... yet.