Here was my thought process in making a decision:

I could get a replacement used HR10-250 off of eBay, but I don't trust eBay that much, I'd be getting a used machine, and I'd be spending $300-$400 on a machine I know for a fact will be obsoleted in a couple of years at most. Also, I get limited HD access from DirecTV, in particular, no local HD channels, as they're all on MPEG4 streams. I've been recording OTA, but I'm not very close to the transmitters, and I get frequent dropouts.

I could get the DirecTV HD PVR. But it's not as cheap as it initially seemed. I'd also have to get a new dish, which would be a couple of hundred dollars. Plus, Amazon's reviews (which, barring some outliers, seem pretty good on the whole) give the box just scathing reviews, ranging from multiple swapouts, to frequent reboots, to failing to record content for no reason. And I'd most likely have to get a new 2-year contract with DirecTV. And DirecTV goes on and on about needing a land line. I have VoIP and the HR10-250 had never been able to make a call with it, so I don't know if it would work for the HR20. Not to mention the frustration training my wife on the new UI.

Or I could get the Series 3. It'll require dropping DirecTV in favor of Time Warner Cable, and the monthly fees will be slightly higher, but it's a TiVo. The reviews for the box are outstanding (other than a few people who give it 1 star and say "why would you spend this much when you can get the cable company's DVR for next to nothing", which is a reasonable viewpoint, but not exactly a review of the box). I'll get local HD. I'll get a real TiVo UI. Amazon UnBox movie rentals seems very cool. I don't need a phone line; it'll work over my Internet connection. I can supposedly download shows to watch on my PC. Really, the only downside to this is the cost and the necessity to switch providers. And the sale on Amazon plus the rebate (even though I hate mail-in rebates) puts the cost of the unit in the same neighborhood as the other options.

So I ordered a Series 3 for overnight delivery ($2: gotta love Amazon Prime). Turns out I have to activate the TiVo service on or before the 16th in order to get the rebate, so hopefully I can do that without having cable service.