It kind of depends on what you're shooting. If you are a full-time studio/commercial pro, you probably need a reliable tethering solution, which is something fairly deliberately not part of the Pentax paradigm at the moment. You will also likely prefer to have electronic LS lenses (the Pentax has LS lenses, but they have to be manually cocked each shot, as far as I know).
On the other hand, if you are an outdoor/landscape shooter, the 645D is a much more compelling buy for numerous reasons, including the weather sealing, battery life, screen quality, user interface, lens line-up, WEIGHT of the system, etc.
Moreover, as Yair and others have wisely said, you need to actually use both system and look at files you shot yourself to know. Anything less is empty talk.
- N.