Are you sure you have a 500c instead of a 500c/m? The serial number on the body indicates that it was made in 1972, which is a couple of years into c/m territory (the 500c was discontinued in 1969). I've heard that there are some 'late' 500c marked cameras with the interchangable screen (really just early production c/m's).
Your 501c was made in 1995, according to the serial number. So you can see, it is a much newer camera. Since the screens are interchangable, you can test which one you prefer and keep that (the 501c would have come with a Accu-Matte 'D' stock screen which is much brighter, but it could have been swapped out at some other point).
Honestly, the 500c/m and 501c are almost the same camera, except for age and the inability of the 501c to take alternative winders knobs. I personally love the meter knob, but it's accuracy is no comparable to a digital light meter - it is a (well made) older selenium style meter. It is plenty accurate for black & white negative, but if I were shooting transparency or highly detailed work, I would use something else. Most of the meter knobs you see on the used market are no longer functioning properly, so be sure to inspect and test before buying one.
Again, condition is everything, but the 501c is much younger, so unless you plan to use a different knob or prefer the chrome of the 500c/m, I would stick with the 501c.
Both sets of barndoors look better than my 500c from 1968, and I've never had a single problem with it... so I suspect you'll be just fine with either camera.
Honestly, I'd find it hard not to keep both to have one as a back-up!
