I have two choices of format: 135, 645. I print 35mm negs at 5x7 on 10x8 paper and I print 645 negs at 10x8 on 10x8 paper. So the only difference is really the size of the border.

My main camera is a 35mm P&S. Given this, a 35mm SLR is step up in terms of lugging weight and 645 is even more so. In order to justify the hassle and discomfort of carrying heavy stuff (bear in mind I hate carrying anything) I've got to have something in mind. My logic is usually like this:

Speculative shooting (carry everywhere camera): P&S
Something interesting will probably happen (nice light, you never know your luck): SLR
I'm going to shoot something specific that will be printed (I'm going to do a portrait session): 645