HD and S3 can take any esata drive, but the tivo software won't do it automatically on the HD for anything besides the supported esata drive. Anyway, it's still doable, just not as easy. I don't remember if you have to open the box or not, but if you do, you might as well just install an internal 1TB drive and forget about expansion via the esata port on the HD. (Once you expand the internal drive, I don't think it will do any automatic expansion via the esata port any more.) Anyway, there's an esata expansion FAQ in the tivo community forums in the S3 forum there. You can go read that if you want all the details.

HD supports M-cards in multi-stream mode. S3 can only do one channel per S or M card.

If you really want the backlit remote, you can buy it separately.

Chichow, if you need to use a single M-card, you will need the HD. They could send you two cards (M or S) if you get the S3 though, but they may charge you more for it.

Anyway, there are benefits to both the HD and the S3. I don't remember all the other differences. IIRC, I think the HD supports more total disk space than the S3. The S3 may have a 2 TB limit.

I have an S3, since before the HD was available.

Also, for the HD and S3 (and most modern tivos), only the Tivo USB wifi adapter is supported. They dropped support for other usb network adapters. I have it and it works fine. As stated, you can find it discounted on amazon.com. Both of these also have built in wired networking. That means that if you have a spare wireless bridge around (or one of the routers that can be turned into a bridge with a firmware change), you can also use that, if you need wireless.

-David