Horses for courses. If my end product was 11x14 or smaller I would choose a 35mm camera. But if your intention is indoor shooting as you mentioned "My intention is to use this camera handheld and with enough speed to be used indoors." and it revolved around portraits or such then I would think about a MF body. The 35mm is obviously your multi use tool that covers weddings, music concerts, children and action work easily besides lens stabilization that came latter, and 645 does give the bigger neg, but in the end the 645 is just a awkward handling body to me compared to a 6x6. I owned a 645Pro and it always seemed an between format that suffered inadequacies in speed, TTL flash and metering to the smaller size, and in format size to the larger formats such as 6x7 and 6x9; A middle child more or less. Of course this was my impression and there are many who will sing it's praises. I say buy one and find out. All that will happen is you will have to sell it. Btw, I owned the 55mm for the pro and I was mostly un-impressed at the negs. The 80 was nice. If I were to go D at some point your older 35mm lenses will work in the brand except in Canon.