I currently own a Nikkor-O-C 50mm 2.8 (multi coated) and a Nikkor-H 50mm 3.5. I used to have the Nikkor-O 2.8, similar to the Nikkor-O-C, but single coated. My impression (I only got the 3.5 this summer) is that the 2.8 is sharper, at least wide open, and that the 3.5 shows more obvious vignetting. The 2.8 is very sharp, and I often use it wide open.
The single coated 2.8 is noticeably more prone to flare than the multi coated version, but the original hood is quite effective. I all depends on the lighting conditions.
Bronica also made a Zenzanon MC 50mm 2.8. A multi coated and very compact lens, which uses 67mm filters. I've never used, or seen one, myself, but it's supposed to be very good. I think it must be quite rare, at least compared to the Nikkor 50mm lenses, as they don't turn up for sale very often.
Trond