There is no Series 2 HD Stand Alone TiVo. The two HD capable TiVos are the Series 3 with THX and the TiVoHD, which is based on the Series 3 architecture.
There is an HD DirecTiVo that is technically a Series 2, the HR10-250. But this only works with DirecTV.
The main differences are:
- The Series 3 has THX certification.
- The Series 3 has a 300 GB drive, the TiVoHD has a 160 GB drive.
- For now, the Series 3 can only use Single stream Cable Cards (S-Cards), so you will need two. The TiVoHD can use either S-Cards or M-Cards (multiple stream cable cards), so you will only need one M-Card or two S-Cards.
- The Series 3 lists for $599, the TiVoHD lists for $299.
- The TiVoHD cannot use an external hard drive connected to the eSATA port to store shows. The Series 3 can, but only through using a kickstart code.
There are other differences that have been outlined in the linked page.
You will get more relevant answers if you post your Series 3 related questions in the Series 3 Forurm on this site. I answered here because I also own a Series 3 TiVo.
Hope that helps,
robomeister