At this point in their careers Brad is the better rebounder (Reef is just bleeping embarrassing, Brad merely sucks). Reef is the better post defender. Neither can help defend worth a damn, but Brad may actually have a slight edge because of those 3 inches and extra width -- can get an accidental block here and there just on length.
Reef is a small PF who started his career as a SF. He's not physical -- he's a finesse player. He's completely dwarfed by the serious bigs in there. He can score in the post, which is nice. But so can Ron -- in fact that is where Ron should really spend most of his time. Post play is essential for any serious push, but post play includes rebounds + defense. And its actually fairly rare for a top team to have more than one primary post option on the floor at once. Miami last year won it all with Shaq as the only post playing starter, and then Twoine off the bench. San Antonio with Duncan as the only true threat (Nazr a minor second option if there ever was one, Rasho not at all). Detroit with Sheed, but Ben not an offensive player, and Tayshaun at that time just developing post moves. With Ron in our lineup you COULD start Reef offensively, but you certainly do not have any great need to. And of course the problem with Reef starting is that even in place of Brad he puts more pressure ont eh shot distribution of the team. Brad can function as an outlet with his weak little standstill jumeprs, and get his shots without interfering much with Ron/Mike/Kevin. Reef on the other hand needs the ball intentionally thrown to him with the idea he scores. He's not an outlet option, he's a primary weapon. Its not nearly as unmanageable as repalcing Kenny with Reef, but it still unbalances the team. Reef is largely worthless unless you feed him, and if you feed him in the starting lineup then you are taking from Ron, Mike and Kevin. He, and we, are still probably better off with him off the bench where he can be the primary option and get his shots without interfering with our other primary guns. Meanwhile, offensively Brad is a classic support player, better playing with other primary options drawing attention away from him than he will ever be off the bench, where he just lacks forcefulness.