I've actually only used my 35mm once since I bought a medium format camera, and the only reason was because my 6x6 (then a Bronica S2A) was getting repaired. When I want something grainy I use 3200 speed film, which actually doesn't look too grainy (I only print approx 7x7) but is does have an atmospheric look to it.
Honesty though, the main reason I prefer 6x6 is it just feels better. I love composing with the waist lever finder, the size of the camera, the shutter speed and aperature controls on the lens. I have also found that the square works wonders with my imagry.
6x6 just works best for me, but you'll find so many people out there who say 35mm works best for them, or 4x5 or 8x10 (or the really crazy ULF shooters!) and even <gasp> digital cameras. I'm happy to hear that you've found a format that you really enjoy that also supports the imagry you're looking for.