no one has mentioned enlargers... do you do your prints yourself? Have you budgeted for one (and lens) that will work with whatever you choose?
I've got a Mamiya 645 and the lack of interchangable backs on my model is annoying. When I looked at other options I liked the Bronica GS1 (neg size, weight and size). I just bought a 4x5 capable enlarger so might stick with the Mamiya and get a 4x5 instead... decisions.. decisions!!! I was hanging onto a 6x9 capable enlarger just to be able to print some old family negs but that will be able to be off loaded now
As far as enlarging goes... forgetting about grain which may not be a problem, I find the larger negs hit the paper nicer... tonality seems better and I'd say I do less dodging/burning than with 35mm ones. I put it down to the lower levels of magnification.