The biggest differences between the 6 and 7ii as I see them:
(1) As mentioned, the lens family for the 7 series is larger, as mentioned;
(2) The 6 is a bit more compact and I really like the collapsibilty for travel, it packs very well and securely;
(3) There are significant metering differences; metering on the 6 is kind of ambiguous- it is either full frame or kinda centre-weighted depending on which lens you use. There are workarounds and tricks though;
(4) Double exposure- can't do it on the 6. Usually not a big deal but recently, in the desert, I really wanted to preflash an exposure. Then I remembered that the 6 cannot do multi-exposures, so I would have had to preflash the roll externally before loading it;
(5) I see basically no difference between 6x6 and 6x7- for RF shooting, I like to shoot square(ly) and I always aim to preserve the ratio and the whole neg/chrome in the print. If you routinely want to shoot for a final 6:4.5 or 6:7 or 6:8 aspect ratio in the print, then the 7ii will deliver ultimately better crops. But as for me I like squares and find them easier to compose.
Anyway... just some stray advice: whatever you get, plan on getting to two bodies before long. Things can and do happen and, anyway, it's a big luxury to be able to load one kind of film in one body and something else in another. If I have any complaint at all about the mamiya RFs, it's that the backs aren't interchangeable on the fly... I know I know, I want too much.
All that said, I have two sixes and treasure them, and am now looking to pick up a 6MF as well.
Other things equal I'd buy the 7 because there's more stuff around if you need to replace something and a much greater ease of getting spares. There are things that are known to go go wrong with the 6 that are difficult to get fixed because all the spares have gone. Of course if it never breaks all this is irrelevant. But the 6 is an old camera now, is that a fair assumption?
I wouldn't personally worry too much about the slight format difference. Framing isn't exact and its likely you'll be making some crops anyway. If it were a slr with an accurate finder image, thats a different story.