I prefer the 921/721 type interface over all the other dish receivers. If you have a HD TV then, like Simon sez, the question is almost moot. It should be the 921, hands down. PS. The 921 almost works darn good now.. PSS. You could always get both.
The 921 is not as unstable as you might think. It used to lock up and reset itself for no apparent reason several times a day for me. I haven't seen that problem since the SW update that reboots the 921 nightly.
It isn't something end-users can set-up/change, it was included in a software update L1.86 or L1.87. It will only reboot at night if the 921 is in standby and there are no event timers set to fire near the time of the reboot.