The HR21 also has two network jacks(RJ45). One is for pass through. If you have a second DVR, you can connect into the second jack on the HR21 and not have to have a home run to the router.
The HR21 also has a 320Gb HD vs the 300Gb in the HR20.
The HR21 has Digital Audio Coax as well as Optical. The HR21-700 only has Optical. IIRC the HR20-100 has Coax.
The HR21 is gloss black and the HR20 is matte grey.
Functionally everything else is pretty much the same. However, sometimes the firmware for the HR21 is a step behind the HR20.
IMHO, the HR21 is much quiter than the HR20.
Mike