Pentax 67 vs. Mamiya7II: which would you get and why?

For what it's worth, both are huge. I mean, leviathan gigantic. I did not realize just how bulky the Mamiya 7 was until I held it, and it's certainly no Olympus XA. The Pentax 67 is larger.

To be honest, I don't have much experience with either -- I've seen a friend's photos from the Mamiya 7 and I've held it. The Pentax I've shot for a day, but much of the film was lost along with my messenger bag. For the Mamiya 7, I have to say that it is sharp. Granted, my buddy always makes beautiful photographs, and I will never be half the photographer he is, but there's something to be said about the razor sharp photos from the Mamiya. Test charts and all that garbage aside, a sharp lens adds that extra crispness at all sizes and viewing distances and ought not be underrated. But, both he and I agree that what the lenses possess in sharpness, they lack in pop. Photos out of the Mamiya 7 are a bit dull -- not terribly so, but where a Rolleiflex will drip with contrast and saturation, the Mamiya seems a bit... mundane. No, I can't back that up with any tests, it's just something that I've noticed. You will have to learn to deal with parallax error.

The Pentax is a nice rig, I dig the larger lens system and all that, but I don't know that it's practical to shoot handheld. I mean, yes, you can shoot it in hand -- you can shoot a Mamiya RB67 in hand if you so desire (and hate yourself). It's just not very practical, you will find that you need lots of light and the shotgun-blast of mirror slap will decrease sharpness to some degree at every speed. Were I willing to handhold a Pentax 67, I'd just go for a Hasselblad with a prism.

There is also the Pentax 645, which is the closest to the ease of 35mm of any medium format system. Fun stuff, the Pentax 67 lenses can be used on the 645, so if you find that you need to use the camera handheld, you could always just swap out for a 645 body for the day. The Bronica RF645 is my favorite medium format rangefinder -- it is just a beauty to hold and operate. A Rolleiflex will limit you to one lens, but you will appreciate the camera when you shoot it. The Hasselblad 500/501/503 series cameras are relatively light and easily handheld, plus they have a range of first-rate lenses. But were it my choice between the two that you listed, and if I had no intention of macro photography, I would snag the Mamiya 7II in a heartbeat.