There's something to be said for square. It's big enough, especially with today's films, to crop at will to a rectangle without having to rotate the camera or the back, essential for TLRs and handy for MF SLRs with waist level finders. The trade off is wasting a bit of film compared to a 645 camera. Of course I have some images I composed and shot square and like that way, but you can just as easily crop the 6x7 back to 6x6 when you want square.
I have both 6x6 and 6x4.5 cameras (TLR and SLR respectively.) I will happily use either handheld. Having handled an RB, while you CAN shoot some with it handheld, it's just not a practical walking around camera. If you want that in 6x7 get a Pentax or a Mamiya 7 (and be prepared to pay for the latter.) Or possibly one of the Fuji RFs if interchangeable lenses are not a must. The Pentax is more workable handheld than the RB but you give up interchangeable backs and the rotating back, and it's still bigger and heavier than the Hassleblad.
I come right back to what I said before - if one is ONLY going to use it on a tripod, get the RB. If it's going to be used handheld, even occasionally and especially while moving from place to place like walking down the street or around at an event, get the Hassleblad (or something else, but not an RB.)
By the time you crop a 6x6 down to the same aspect ratio of 6x7 you have more than 50% more surface area with a 6x7 frame. My GS-1 isn't all that much bigger than my Rolleiflex SLX. Probably why the Rollei is rarely used.
That's true, but does it matter? It depends on your final print size, your film and developer, and your tolerance for grain. I happily print up to 16x20 from my 645 negs, or 6x6 cropped to 645. The bigger negative would primarily be an advantage to me for cropping further for composition or to make up for a lens a bit shorter than I would have preferred.
If you print larger, use film that's grainier, or are averse to any visible grain at all then it starts to matter more. I can certainly see the difference between 645 and 6x7 on a 16x20 from fast film, at least if viewed fairly close. More commonly I print 11x14 and at that size the difference becomes very small.
I totally understand the appeal of the lack of grain and smooth tonality of a larger negative enlarged less - I do shoot 4x5. I'm not familiar with the GS, at least in person (never seen one IRL.) The RB is just bigger and heavier than I, or many people, want to carry and use handheld so if the choice is between the RB and Hassleblad it comes down to that. Now if I'm going to use it on a tripod all the time, I'd take the RB hands down for that bigger negative. I'm not a believer in the church of Zeiss or any other magic lens make. Good lenses, yes, special magic in certain brands, no. Mamiya glass is excellent.