Ok, Physically and software wise, there is NO difference between the 508 and the 510. Other than the DVR fee, they are the exact same receiver. You can replace the hard drives in the 501 and 508 and make it a 510, and you don't have to pay the VoD fee, and you get the same recording time. However, this requires someone who knows what they are doing.
If you are looking at an E* PVR, don't overlook the 721. It has 2 tuners like the 522, but only outputs to 1 TV. However, I must say that having BOTH a 501 and a 721, I rarely use the 501. The 721's guide displays much more info and is much faster than the 5xx. And, it has nice touches like showing the current channel when you bring up the menus and PVR list like the 522 does. Add the 90 hour recording time and the built-in PiP, it's a heck of a receiver. Granted, there have been some quirks since the L175 update(most notably the skip back 10 second problem), but it is still a better receiver IMHO than the 5xxs.