Most of the reasons that Comcast isn’t my ISP have to do with its aggressive opposition to net neutrality. But there’s also a substantial quality of service issue: see, for example, Steven M. Bellovin, An Outage from Managing P2P Traffic?.
Not that I love AT&T, or that their net neutrality politics are so much better. But their service seems somewhat better, and they’re a little less in-your-face about their views.
Michael, follow-up messages indicate this was just a glitch. There may be real differences in service, but that can’t be determined from the alternate hypothesis that this is just a bad cherry-pick resulting from the recent demonization campaign
(i.e. every Comcast bug is reported with a paranoia-stoking take of ZOMG! P2P!!!).