I started off with a Magellan Explorist 210 and used it for about two weeks to find about 18 caches. I decided that the 22MB of internal RAM wasn't enough, so I returned it and got the 500. I like it a lot more! It has a color screen (I didn't think I'd care, but is looks way better) and an SD card slot, so I can store most of the USA in detail maps on a SD card. It also has better software features and slightly better accuracy. I like the ability of the 500 to mark a downloaded geocache as "found." (the 210 didn't do this).
In case you don't know, the 210 and 500 (including the LE) (and 600, XL and even the 400 I believe) can be plugged into your PC and you can transfer geocache waypoints, other waypoints, routes, maps, etc. to the device.
