Steve:
O.G. pretty much nailed the major differences.
I have owned both cameras over the years. The 503CW is obviously the more up to date version. Both cameras are all mechanical. The bodies are the same size so that you will not be limited for film only, the digital backs that are made for the V System cameras will fit on either body, and require batteries to function, and are part of the digital back. Any film back that was produced in the early years as well as newly produced backs will work on the 503CW and the 500C/M.
One other minor difference is the viewfinder screens. The 503CW comes with the newer bright viewfinder screens, whereas a 500C/M screen is somewhat dimmer. However, the 500C/M version allows for changing the viewfinder screen so you can get the newer bright screen and easily replace the dim one.
Ron