Both are popular in this hobby. It totally depends on the model in question.
Rogue Audio claims they broke up the Zeus 225 lbs stereo tube monster into Apollo monoblocks because of the logistics of moving the damn thing. When you have a single chassis weighing beyond 100 lbs, it gets pretty difficult without hiring help - especially secondary market sales w/ shipping. Monoblocks help break that up, though then you're paying for shipping 2x boxes, 2x power cords, 2x amp stands...