Unfortunately, no, the SA8000 and SA8000HD aren't as responsive as the products from Tivo and Dish Network, in terms of both channel changing and and PVR functionality. With the latest software releases, the SA8000 isn't nearly as bad as it once was, but the Comcast in your area could still be using an older software version.
That said, the hard drive on your SA800HD could be going bad, or it could be fragmented, making it even less responsive than normal. I would drop by your local office and swap it out with another. If the replacement isn't much different, then the standalone Tivo would be an excellent choice, particularly if you only subscribe to analog cable (extended basic). The SA Tivo has a built-in analog tuner for extended basic cable, but relies on IR remote signals to control and record channels from a digital cable box.
Because of the way Tivos work, and what they let you do, it's generally preferable to have extra capacity. You can buy the 40-hour model (which is actually only ~12 hours at the high quality setting, iirc) for $79 and then replace the drive with a Maxtor Quickview 160Gb for about $120. You would first need to setup the drive with a PC using the instructions over at TivoCommunity
. If you don't want to mess with drive setup on a computer, you can purchase a pre-setup 160Gb HD add-on kit with mounting bracket from Weakness
for $190. This is still a lot cheaper than buying the 140 hour model from Tivo for $399, minus a $50 mail-in rebate.