Can't help you with which one of those is better, but I recently had an X-100-B restored by Paul Grzybek of Tube Audio Design and I can highly recommend him. He did a great job and the amp sounds incredible.
5 responses Add your response
According to a copy I have of a Fisher ad from that period, the 800C and the 500C were identical except that the 800C included an AM tuner section and the 500C did not. Perhaps the 800B and 500B differed similarly, except for the "B" versions being a little earlier.
According to an old bluebook I have, original list prices (not necessarily in the same years) were:
The 500C and 800C are indicated in the ad as having "a new Nuvistor-Golden Synchrode front end (for FM) affording far higher sensitivity, a wider overload margin, and a better rejection of spurious and imaging signals." Based on that, if you aren't particularly interested in having AM included my instinct would be to go with a 500C rather than an 800B, if that is the choice you have.