I say, it depends. If you live in the cold north, with many winter storms and heavy spring rains, be prepared for interrupted signals.
Otherwise, I think it's a wash when it comes to cost and programming. I'd give DirecTV the edge on DVR choices and features as the Comcast DVR (Motorola) sucks balls and is just barely functional when compared to a Tivo. And don't try to get a Tivo to work with Comcast, as they have no interest in providing efficient tech service and support if your not renting their crappy box.