I know this is tin-foiling it a bit, but I'd say another advantage of a Tivo over a Comcast owned box is that the Tivo is providor independant. If in the future Comcast doesn't want you fast forwarding through commericals, it's only a software upgrade away.
Now Tivo could do the same thing, but I doubt advertisers would be able to lean as heavily on them to do it. It'd likely take some sort of legislation or lawsuit for Tivo to cut out their most popular feature.
Comcast does have a nice price point there. There's Tivo2Go, home media options, and remote program scheduling over the internet with Tivo, not to mention Season Passes and the Tivo suggestions. You'll have to decide if that stuff is worth $80 for the tivo and an extra $5/month.