Time to go HD on our 2nd sat reciever, which reciever?

Well, there is only one: HR21.
HR20 is the older model that has OTA tuners built in but rare you'd get that one.
HR22 is the newest model, just released a couple weeks ago. It's the same as the HR21 but with a larger hard drive. Again, rare that you'd get that one either from an installer. You can get them at Best Buy though.

Functionally, the HR20, HR21 and HR22 are pretty much the same. The HR20 has built in ATSC tuners for OTA while the other two require an external tuner (AM21) for OTA. The user interface, menu, guide, etc., are the same on all.

If you are doing the install yourself (plug and play by replacing the R10), you can pick one up at Best Buy, Circuit City, Costco, or from several on-line dealers. You might have to order b-band converter modules if they don't come with the DVR. By doing it yourself, you have control over which specific model you get. By ordering from DirecTV, the installer will show up with whatever is available.

Regardless of where/how you get the unit, it will be leased.

Carl