Quick question, longish answer, trying to keep it compact:
Compared to the 500 C/M, it is almost not different at all.
The 500 C/M has a detachable wind knob/crank. The 501 C hasn't.
The 500 C/M has a body ready signal window. The 501 C hasn't.
The 500 C/M has a locking lever around the shutter release button. The 501 C hasn't.
The 500 C/M has the old style tripod foot, the 501 C has the new style foot (still fits the old quick couplings, not the other way round).
The 501 C also has the new style two ridge support bottom. The 500 C/M does not.
Apart from what the older camera may (!) have experienced to make it worse, neither of the two is better or worse than the other.
The "etc." bit of your question requires a really long answer.
But quickly, limiting the list to functional features that really make a difference:
- The 500 C/M replaced the original 500 C.
It has user changeable focussing screens (the 500 C had a fixed screen).
- The 503 CX replaced the 500 C/M. It has a sensor built-in that can be hooked up to flash units through an adapter to provide TTL OTF flash metering.
It also came with the Acute Matte screen: brighter, but not really better. Screens can be changed very easily. So do not trust that a camera has a certain type of screen in it based on the camera type alone. Always check!
- The 503 CXi replaced the 503 CX.
It differs from the 503 CX in that it has a different wind crank mount. Not yet available when this camera was introduced, so we did not have a clue what this change was for, but it means the 503 CXi can be used with the Winder CW.
- The 503 CW replaced the 503 CXi.
It differs from the CXi in that it has a bigger mirror. The older cameras all have a mirror that is a bit too short, and causes viewfinder vignetting with longer lenses and when using extension (tubes, bellows). This only affects the viewfinder image, and is something you get used to.
The mirror was made too short to make sure it would not hit the rear element of the 80 mm lens when swinging up. The larger mirror in the CW avoids the lens by moving backwards while moving up.
The 501 C and 501 CM are a separate line, also branching off the 500 C - 500 C/M line.
- The 501 C is described above. Basically, a 500 C/M without a few minor features.
- The 501 CM is a 501 C with the larger mirror.
They are all (!) equal in quality.
No single model is better or worse than any other. No model to prefer or avoid for any other reason than the extra feature it may have.