Since soon you will only be able to buy new either the H1D or the 503CW then I think it will be much easier to decide.
The 555ELD is the most "complete" of the 500 series of cameras. All the rest "miss" something from the former.
The 503CW misses the built in winder but has an optional one. Plus it is not as "digital ready" as the 555ELD (whatever that really meant).
The 501CM misses the TTL metering of the 503CW (and the asa dial) and the winder.
The 501C misses the gliding mirror.
And the rest of the 500 miss the gliding mirror, the acumate screens and the extra stray light reduction inside the bodies.