Hard to say, you'll just have to look at the pros/cons and decide.
I'll start:
PRO: 811 is free! (to new subs)
CON: 921 is $1,000 to ANY sub.
So if recording HD is worth $1,000 to you, then get the 921.
If you just want to record SD satellite stuff, you could also get a 522/811 for free with your installation.