Anyone recommend which model HD DVR to request from Direct TV?
I have an HR20-100 and an HR21-700.
I don't think you will get a choice. You will get what ever they have. Both these units are very similar, but the HR21-700 has a significantly shorter reboot time.
I can tell you one thing, a UPS IS MANDATORY with those units.
My HR20-100 always was on a UPS and gave no trouble. Originally I had two of those, but I had the one not on a UPS and it locked up continuously and was needing reboots ridiculously often. Two of them in that location failed, and the last failure also took down the electronics on the roof, so the other one would not work.
When the third unit, the HR21-700 was placed in that location, and new electronics placed on the roof, I added a UPS at the second location. Since then lockups have been very rare, I have had two instances between the two of them in the last year.
I found these systems are very vulnerable to very small peaks and especially sags in voltage.
You will absolutely need to figure the cost of a UPS in the package.