There is no right or wrong answer really.
Here in southern Michigan where I'm at, the answer is obvious -- avoid Comcast at all costs and go with SBC. However, there are areas where Comcast probably is better than SBC DSL. It varies from area to area. I use SBC DSL and almost never have outages, and never have download speeds drop from just below the 1.5Mbps I'm supposed to get. I'm quite happy with it here though and I'm paying $25/mo. for it. I recommend SBC to everyone locally that will listen. Your results may vary though, depending on the area.
As to Comcast's "We're broadband, DSL is not" claim... they're simply trying to twist a definition to make themselves look better. Technically they're correct I suppose -- "broadband" generally refers a high-bandwitdh service that carries, data, voice, and video all over the same line. But DSL doesn't do video like cable does (yet), so technically by that definition DSL isn't "broadband".
HOWEVER... you're not asking about video and voice end, you're asking soley about data service... which renders their claim and point completely moot if you ask me.
The actual general public definition of "broadband" is generally "anything exceeding 128kbps ISDN" though. And by that definition SBC DSL fits because generally on a good 1.5Mbps connection you should be getting somewhere around 1250kbps steady on the downstream.
* Cable = whatever speed promised that you'll rarely, if ever, reach. Could still be faster than DSL though, but it's a bit of a gamble dependant on your node.
* DSL = lower promised speed, but steadier and you're more-apt to actually reach it. Assuming you're not too far from the CO, that is...