The 500c is older, and usually slightly cheaper. The difference is that with the c/m the focusing screens are easily interchangable, and the A backs will automatically wind to the first frame and stop. With the c you have to watch for the #1, and stop winding on your own.
Just a couple of points of clarification in case somebody down the road considers a Hassy:
1.) The 500C and the 500C/M (and 500EL and 500EL/M), only major difference is the user changeable screen. Other than that, they are exactly the same (though earlier C's have a body sync connection that fires when the rear curtains are fully open) I use a 500EL frequently, and I have no problem using it, in fact if durability is a consideration, my understanding is the transport of the EL/ELM is significantly more robust. And for about $5 you can convert them to run on a 9V battery.
In my opinion, the advantage of the Hassy is the extreme precision that they are manufactured/adjusted to, however that is also their biggest downfall, as they require regular maintenance or they get really finicky.
2.) Any back will work on any camera in the 500/200/2000 series, but not the 1000F/1600F which have their own backs(though they look almost identical) The original style backs (12, 16, 24) have a peephole, and an optional plug for 220 use. (no backing paper) The later "automatic" backs, typically called A12, A16 and A24, allow you to align a mark on the roll with a mark on the back, put the insert in and it will automatically stop at frame 1. After loading, there's no difference in operation. Personally, I prefer the A backs, as they tend to be in better condition. Plan on replacing the light seals on any back you buy, they're cheap ($12 from VH USA, don't bother with the scam artists on
ebay) and if you are capable of minor mechanical work, they are simple to install yourself.
I'd suggest any potential Hassy buyer buy an used copy of the Hasselblad Compedium and the Wildi's Hasselblad Manual (earlier editions have more coverage of the older systems), inexpensive on Amazon, and will help you with the finer points and gotchas that can crop up.