I have both the Pentax 645N and Pentax 67II. The 6x7 doesn't get as much use because of the bulk of the camera. I like using the 645 more and it has autofocus for my bad eyes. But when I go out on a really important landscape shoot, I'll bring both. One for color and one for B&W. Depends on my mood which one will be for B&W. But my 67II eats batteries, so it tends to sit with batteries out a lot.
I also have 2 4x5 cameras (don't get any use lately) and several 35mm cameras. If the shoot is just for fun, I'll bring the 645 for color and 35mm for B&W.