I have used both Comcast and AT&T and AT&T is no comparison here in the bay area. AT&T advertise 3mpbs or even 6mpbs, but their backhaul is not as fast as Comcast (backhaul - the actual interconnection to the "rest of the internet"), so you may not able to get full 3 or 6mbps when connecting to servers attached to other backhaul providers, basically the rest of the internet.
With DSL, you also have to worry about the condition of your phone line, so it's very possible your modem is not able to "sync" at the 3mbps or 6mbps. Even if your phoneline is very clean and you get the full "sync" rate, you still will not get the full speed. Combined with DSL PPPoE or IPDSLAM overhead, you will net about 85-87% of usable bandwidth if you are super lucky, whereas Comcast can net very close the rated speed.
To put it in practical term, I just switched to AT&T DSL Elite, 6mpbs/768kpbs. Lucky me, I have a very clean phone line, which can support much higher sync rate than Elite, so I am getting maximum performance. With the DSL ATM/PPPoE overhead, I am netting around 86% = 5.12mbps down and 0.60mbps up on speedtest.net. You would think this beat's Comcast's? NO!!! I put my mom on Comcast's Economy Internet, 1.5mbps down and 384kbps up, speedtest to be 1.48mbps down and 0.37mbps. These numbers would lead to you believe AT&T would be better? NO NO NO! I can't stream Netflix or similar service on AT&T without constantly stuck in "buffering", while my mom on a slower Comcast service can breeze through the entire movie without disruptive buffering.
A lot of streaming contents don't exceed 1mbps, unless HD. While AT&T's connection to me may be 5mbps or so, their backhaul network is so slow, it never fills up the 5mbps or even come close to holding steadly of the 700-800kbps needed to stream Netflix well. However, my mom's Comcast may only be 1.5mbps, Comcast's intercarrier overhaul is very fast, so they kept maxing most of the 1.5mbps constantly, thus allow stream of Netflix without issue.
DSL or U-Verse, AT&T's backhaul network is really slow, you are better off with Comcast.