I had a Pentax 645n, and now I have one Rolleiflex, one Zeiss Ikonta, and one Olympus XA , also 3 digicams (5D MkII, Canon G12, and the iPhone).
I bought a 500C/M with an 80mm like 6 or 8 months ago, and this camera just blow everything away. I never touched any of the other cameras again. The thing is just perfect and the negatives are A LOT BETTER than the ones I got with the Pentax 645n (which was my previous workhorse). Better in shadow detail, in tonality, color, vignetting, sharpness... everything.
About 500C/M vs. 500C, the better focusing screen is really better. Really. To be frank I think that the focusing screen on 500C really stinks. It's dark and soft... Doesn't do justice for the system and you'll feel frustrated, IMHO.
I just bought a ELX also, to have a backup, and because sometimes I'm working with natural light portraits in the studio, on a tripod, and I have to wait a couple seconds after wind the film to shoot again because winding the film with the lever will shake everything. My kit is the 80 planar, the Variogon 140-280, and I just bought a 50 on fleabay also, which didn't arrive here yet, but I'm praying and it will.
Im selling everything else. The thing is that good. I'll keep just the iPhone and the 5D, and thats because I work with the 5D (costs and sometimes clients preference, go figure!) But If I was just shooting to myself, I would get rid even of the digi fullframe. After Hasselblad I feel like everything else analog is "almost", and digital, which used to be "almost", just became "s#*t".
Buy the 500 C/M.