Ok, don't take it as 100% correct but here are what I overheard and "gathered" from different topics and Ziova posts:
1. Different box ( obivose
2. Different remote (Z500 one was backlight and with "joystick" but problematic and had too few buttons (on my taste).
3. Different wireless card (that one can support WPA, no idea if it's enabled in first firmware).
4. No SCART (not that it ever worked other then in composite mode on Z500 anyway).
5. I believe some parts as PSU and VFD are different to be compilent with RoHS.
6. On the model without DVD there is different recovery from USB procedure.
Since both have same chip inside the memory and such are 100% same (memory limited by EM8620L Sigma chip maximum capacity by something like 16MB).
Can't say "slow menus" , they are about as slow as any other EM8620L based player , the topic arrised because the Z500 menus were very fast to begin with but in last firmware the speed was reduced because of memory/stability issues.
And yes, it's kind of slow
The same firmware goes both to Z500 and CS50x the difference is version Z500 1.6 = CS50x 1.0 and different drivers compiled in because the differences in hardware (WiFi etc).