I agree with Robert, "resolution isn't the only thing out there".
In B&W, I've come to the conclusion that it's one of the least important factors. Tonal gradation is the most important factor. I recall from my fooling around H&W Control film (1971, I think) is that it produced an incredibly sharp, incredibly terrible print.
I think you should pick your format based on required sharpness for a film giving you what you consider to be good tones. You won't lose much spontaneity using a medium format camera with fast film if, to get the same sharpness, you have to load EI 40 film into your 35mm camera.