I'll just offer my opinion that 35mm RFs and medium format RFs are completely different beasts. Yes they are both rangefinders, but that's just about where the similarities end. A typical 35mm RF takes
much faster glass and is more compact. The mamiya RFs, even the collapsible 6, are a bit too big for your pocket.
The mamiya RFs are not particularly suited to portraiture or shallow DOF work in general. However, I think they are brilliantly suited to a documentary style of shooting and landscape. No, they won't fit in your pocket, but they do travel well. And they aren't something that you can whip out of your sleeve on a city street, but in terms of handling ergonomics, they do remind you more of a 35mm SLR than a MF SLR.
The mamiya RFs are also not particularly suited to low light situations- the fastest lenses are f/3.5 for the mamiya 6 and f/4 for the 7 series. So low light shooting is not a particular strength of the mamiya RFs. If mamiya made an f/2.8 RF lens that would be awesome, but alas, it isn't going to happen!
Now, as for printing, indeed there is huge difference between 6x6 / 6x7 and 35mm. Just for reference, I consider my very best 35mm negs to enlarge well to maybe 8x10 or maybe a bit further. In my opinion, the MF RF negs go at least 2-3x as far. For what it's worth, the MF slides/negs are also much easier to scan by flatbed.
I shoot LF as well, mostly 4x5 and more recently 5x7, and my honest assessment is that the mamiya chromes push 4x5. Go ahead... gasp, chuckle, whatever

The reason I say this is that the mamiya RFs offer a really ideal compromise of larger capture size and superb optics. A properly exposed 6x6 chrome or fine-grained neg easily goes further than I care to enlarge.
Regarding b&w tonality, well, nothing beats a LF contact print. But you will be amazed at what MF can do in b&w. I've been doing most of my IR stuff on a mamiya 6 and there is just a hint of grain at normal enlargement. And for chromes, I think MF really pushes LF and the chromes are just packed with detail.
So... how does a mamiya 6/7/7ii compare to 35mm? Well they
don't compare, really. They are very different in many ways. I'd just say that if you are looking more for a wide to normal landscape/scenic/documentary camera then the MF RFs are serious weapons.