No real world differences, other than the 500 has a bug where it can act up with the AM21 OTA tuner. Looks like this is fixed in the newest beta software.
The 100 and 200 also have the Broadcom chip, and the 500 has the NXP chip, which instead of outputting what most DVR's do, ycbcr, it outputs RGB. This can cause issues if you have a HD set that needs a different menu option for RGB, but it's not big deal either way. Just know if you get a 500, the OTA may still be buggy when recording, and it outputs RGB.
Other than the AM21 issue, I love my HR24-500, and it's speed.