I can't speak for the 500 but my gf has a 600 and I have the Nomad II.
The Rio has a better design. It's much sleeker and less fragile. It is formed to fit your hand so when you grasp it is feels nice. The Rio's display is bright and sharp. The menu is quite intuitive and easy to use.
The Nomad II has a more solid feel to it. I think if it dropped it would smash a lot easier than the Rio. The big plus of the Nomad is the remote. I never have to handle the unit itself, you can do almost everything you need from the remote. A big plus when riding the NYC subway system

The memory is SmartMedia, which is a plus.
Rio 600 Pluses
Nice form factor.
Nice display/menu.
Has 32M
internal.
USB port seems less likely to break than Nomad's.
Fast USB transfer rate.
Rio 600 Minuses
No Remote.
RAM expansion is proprietary.
The on/off switch sucks and standby does not seem to work well.
USB cable is non-standard.
Nomad II Pluses
Remote!
SmartMedia (better than proprietary, but Compact Flash would be better).
Standard USB interface.
Did I mention the remote?
Nomad II Minuses
No internal RAM.
Seems fragile, esp the USB port.
SLOW USB transfer rate.
I hope this helps.