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.
I have a couple of HR24s, one is a HR24-100 and the other one is a HR24-500. I have problems with the HR24-500 displaying caller ID, I need to reboot the receiver to get caller-ID working again. The HR24-100 doesn't have this problem.
My HR24-500 has had no caller ID problems but Media Share is essentially unusable (audio skipping and dropouts). It didn't have this problem until a few months ago (there were a few minor Media Share issues). I moved my AM21 to my HR21 so don't know if the issue with OTA recording has been resolved.
It seems the two problems with the -500 here are 1) caller ID and 2) OTA with the AM21.
I just ordered the HR24 from Solidsignal to replace my dying/dead HR20-100 and those two problems won't affect me.
I do have the AM21 on my HR23 and was thinking of moving it to the HR24, but it can stay with the HR23.
And for caller ID, I use it on my cordless phone handset.