I have taken two SI courses, and found them both to be outstanding. I have not taken any Gunsite courses. One difference, as I understand it, is that Gunsite absolutely followes Cooper's Modern Technique which, among other things emphasizes aimed fired, (or flash sighting as the situation warrants) without exception. SI is more flexible in philosophy, teaching point shooting, flash sighting and aimed fire as the circumstance warrants. Point shooting for Gunsite, as I understand it, is pretty much considered a no-no. Of course if we have any Gunsite alums here, please correct me if i am wrong.
Of those two Id definately go for Gabe. They teach common sense, aggressive tactics that fit real life. Gunsite? Watch the Wed night shows. They fire twice and wiggle the gun back and forth (that's "assessing"). Rob Pincus would be on my short list too. Same modality--common sense advice and Rob is a thinker.