The problem is that the 645 must be tilted on its side to get portrait orientation, the RB has a rotating back that allows the camera to stay upright. That makes the RB much easier to use than the 645 in portrait orientation.
Depends on the 645. I'd much rather turn my 645 Pro with its winder grip on its side for a portrait shot than to hand hold an RB. The Pentax with built in grip and the ETR with a grip are probably as good or better.