Comcast owns the local cable monopoly, and is the only provider of terrestrial high-speed Internet in the area. Comcast is a monopoly, and they own a lot of the local infrastructure for transmitting bits over wire, which is generally recognized as one of the best ways of transporting electric binary numbers over moderately large distances fairly quickly and cheaply.
It seems almost brain-dead obvious that the wired carier--Comcast--should have the better Internet rates. At the moment, they're advertising to rates for my area: $24.95/month for 1Mbps down and 384kbps up; or $42.95/month 15Mbps down and 3Mbps up.
With an existing family plan for T-Mobile (voice), one can add a second line for $5/month, and get a flat-rate data tariff for $25/month on top of that. That means that the line would cost an even $30/month--just over the $24.95/month of Comcast's cheaper plan, and well below the $42.95/month of their premium plan (excluding taxes and other fees on each). The T-Mobile plan supports High Speed Packet Access usage at 14Mbps up and 5.8Mbps down--much closer to the higher Comcast tier.
Admittedly, this requires the purchase of a GSM to WiFi bridge, but a Comcast plan would also require the purchase of a cable modem, or a monthly lease, the final rate of which becomes about equivalent.
Additional qualifications: a) Of course this only matters if you actually get a UMTS connection where you live, in-doors. A UMTS repeater might be an option, but isn't always. b) Sending packets over the air will have higher latency in almost all cases than sending them over a wire. Comcast would have to really screw something up for this not to be the case.