My first reaction to this would be- can you even push 3200 to 12800 and still have a decent looking image (not considering the grain)? I don't shoot a lot of highspeed film, and I don't push a lot of film, so I am not totally sure what the results would look like. But if the grain and image quality is the same/similar, then I would say 6x7 simply because I find it much easier to print from 6x7 negs compared to 35mm.
-Grant