Hi Stuart -- Leica lenses are incredibly good; I just got a new 28mm Summicron, so I'm committed to 35 as well. And Zeiss lenses are great, so are others. Mamiya 7 lenses are legendary for quality, and I think the Bronica's optics are either in the same class or close. The f/4 aperture makes it easier to gain that quality, and keep size and weight down.
Bronica is owned by Tamron, but that's recent enough that Tamron might not have had any input on the 645RF's lenses. Or they might well have, and the cooperation led to acquisition, I don't know.
Yes, it's an interesting question as to whether one is ahead by using, say, Fuji NPZ800 in 645 with an f/4 lens, or with Fuji NPS160 in 35mm with f/2... Sheer negative size means less enlargement to get the same size print or online image, and that has inherent advantages... one of which is that dust and lint aren't so magnified. I'm open to differing views on this question, and may do some close comparing, since much of my shooting is indoor office/shop environmental portraits. Here the issue is put to stress with wide apertures. Outdoors in brighter light, the larger camera can use slow film too, and then the equation shifts strongly in its favor.
I've used Pentax 6x7 for nearly 30 years, so I've long enjoyed its large-neg benefits. I don't see a lot of difference in the output due to the extra size above 6x4.5, but I haven't really put it to the test with huge enlargements either! There's certainly no doubt the 645RF is much smaller to carry than the big Pentax, and indeed not much larger than a 35RF. As for 6x6, I guess it depends on how you use it... DO you use the whole neg to make square pics, or do you commonly crop it down to around 6x4.5 anyway? The square has one advantage in not calling for rotating the camera for vertical/horizontal orientation, but I'm inclined to compose to the shape of the viewfinder so mostly end up with square pics. Which is fine... I sometimes crop a rectangular picture to square too!
Pick your poison, as they say!
