If you're choosing between a TiVo Series 2 and the Comcast DVR, then the Comcast DVR wins, hands down. HD is better than not-HD. Without a doubt. The features are a bit different, and there is nothing so reliable as a TiVo Series 2, but it is still SD, and if you've got an HDTV, then you need an HD DVR AFAIC.
If you're considering switching to the HD TiVo, that's another story. The TiVo Series 3 (the one that supports HD) is far better, when it works for you. (In a very small percentage of cases, the TiVo Series 3 suffers from problems that render it substantially inferior, such as partial recordings and missing channels. These problems continue, for this small minority of customers, months after the Series 3 was introduced.) The Comcast DVRs only advantage is that if it doesn't work, you can repeatedly exchange it for another one, until, eventually you end up with one that works reliably.
Keep in mind that TiVo's pricing has recently changed, and if you leave the TiVo service and then decide to go back, you'll be subject to the new pricing.