Before I left I had one of the Cox SA boxes. It was OK. Just OK. I never really had any problems with it. It tuned fine, and recorded what I wanted. Most of the time. There were some problems when watching one HD program while another was being recorded, or recording two at once.
It's just now the prices have come down on Tivos, the CableCard fiasco has pretty much worked itself out, and Cox has increased prices on the DVR service and box rental so I'd only be paying a few bucks more a month. The main thing that the SA boxes have going for them is convenience. I just take the old box to the office and pick up the DVR. Plug it in and it will (most likely) work. Don't have to deal with guying the Tivo, arranging service, figuring out how to get internet to the box, arranging for the installer, and praying everything works.
So, it seems like the HD is the way to go. Is there anything to the THX certification on the Series 3, or is that just a label on the box and not really something I'd notice?