I think the people who don't notice much difference between 35mm and 645 are falling victim of a psychological pheomenon rather than working from empirical evidence. I can understand it, we all have the "yes, but it feels like ____" moments with different things, but the fact is, just do the simple math. There is a huge difference - there has to be. Which is not to say that if it doesn't feel right to you then you are wrong to go a different direction (6x6, 6x7, etc) - after all, how you feel about what you're doing, in my humble opinion, has a huge effect on how well you work with it. If it "just feels right" you are likely to get closer to the results you want, simply because you are in the right "head space"... Just my opinon, though...
Peter