Do you do your own darkroom work? If so, you will really appreciate the larger negative that 645 gives you.

I think a camera with a waist-level finder is the easiest camera of all to shoot hand-held at slow shutter speeds. You can shoot 645 with a waist level finder, as long as your subject has a horizontal orientation.

IMHO the potential for camera shake arising from hand-held use is much greater than the potential contribution from a moving mirror.

The ergonomics of the camera, and how they suit you are, in my opinion, the most important factor.

I do shoot 35mm, and Mamiya SLR 6x4.5, Mamiya TLR 6x6 and Mamiya SLR 6x7 - hand-held when necessary, braced somehow when possible, with a mono-pod if that is what is available and with a tripod when I can.