Jeff, I have a 50 CFi and used to have a 50mm C lens... I think it's the ideal wide angle for the 6x6. That said, a 40 would be nice, but I don't like the wicked distortion and significant extra weight that lens can give. If I had the dough, I'd get a Super Wide with the 38mm Biogon. It has the least distortion, and a proven performer.
Just out of curiosity, what 50 are you looking at? The C lenses are great, but only the later ones have the T* coating. Mine was a pre- T* lens and worked well for B&W, but was a little soft at the edges until you stopped down beyond f5.6. That said, it certaintly had the old look to the pictures. I now have the CFi and it is superb with the floating lens elements, but again had a huge price swing compared to the C and CT lenses. YMMV.
If you get a C or CT lens, make sure the shutter has had a CLA or send it out for one. Parts are no longer made by Blad and can be hard to come by, so making sure it works right off the bat is a good plan. Then if exercised regularly, should give many more years service.