I own both Mamiya 6 and 7 systems and unless I think I'll be needing a super-wide view (i.e. with the 43mm lens on the Mamiya 7) I always use the Mamiya 6. I LOVE the collapsable lens mount. It didn't seem like a big deal to be able to collapse the lens an inch or so into the body, but it does make quite a difference when carrying the camera. I can fit two Mamiya 6 bodies (one with the 50mm and one with the 75mm lens) into a small camera bag. It also seems less obtrusive to have the lens collapsed when carrying the camera around your neck, as I found when riding in very crowded subway trains on a recent trip to Japan. Keeping the lens collapsed when not taking photos also prevents me from using the camera with the lens cap on (yes, I still sometimes do this, embarrassing but true), as when you extend the lens mount you can't help but see that the cap is on.
As has already been mentioned, not needing an auxilliary finder for the 50mm lens is also a big point in the Mamiya 6's favor.
Anyway, they are both amazing cameras (actually, I should say they are GOOD cameras with AMAZING lenses!), but if not for the 43mm, I would sell my Mamiya 7 and just use the Mamiya 6.
In fact, I think I am going to sell all my Mamiya 7 kit except for one body and the 43mm lens, even though I usually find the 43mm is just too wide (I know for some people "too wide" doesn't exist...). On the rare occasions when I find that a super-wide lens is actually appropriate, the 43mm does priduce awsome results.