HR20 is the first HDDVR, has OTA tuners.
HR21 is an HR20 without the OTA tuners.
HR22 is an HR21 with a larger hard drive.
HR23 is an HR22 with the BBC built in (and is the latest unit).
So get an HR22 or HR23 if you're getting one on your own for the larger hard drive.
I just went from an HR20 with OTA and an external HDD for space, to a HR22 using Mpeg4 HD locals only. Very happy with 100 hours of mpeg 4, also enjoying the sleek look of the unit.