Wrong forum (a Hasselblad is medium format, not large format).

A 501 CM (note the M. The 501 C, without M, is almost exactly the same as a 500 C/M), has a larger mirror.
The one in the 500 C/M causes viewfinder (only in the viewfinder!) vignetting when longer lenses are used, or short lenses combined with extension tubes or bellows. The top of the focussing screens darkens a bit.
The mirror in the 501 CM is a bit longer, and doesn't vignet.

Apart from that, there are a few minor differences.

And there is the age thing, of course. A 501 CM is newer than any 500 C/M than you will find, so maybe (! it is by no means certain) in a better condition than a 500 C/M.