Although I'm not the same person who said that AT&T was better than T-Mobile, I have service from both companies, and here's what I've found:
It looks like AT&T Wireless will have higher data speeds by the end of the year, based on postings I've found on HowardForums. I know for a fact that they've turned on these higher speeds in some markets. To the best of my knowledge, T-Mobile doesn't have any plans to support these higher speeds anytime soon.
Every now and then, I have a problem making a connection to T-Mobile's GPRS service. It usually clears up when I retry the connection, and I've only had to call them once to gripe. AT&T Wireless has generally been more reliable, but not by much (and I had to call them once to gripe about a GPRS outage, too). It's entirely possible that some of these connection problems are between my computer and my phone (which I'm using as a modem), because sometimes rebooting both phone and computer fixes everything.
AT&T Wireless had (or is having) some serious customer service problems in late fall 2003. I tried to call and switch from the 4MB mMode plan to the 8MB mMode plan, and the customer service rep said that their system was down, changing the plan was impossible, and no one knew when the system would be back up. More info on the customer service system outage can be found here, but as of this writing, it looks like the outage had gone from about November 4th, 2003 to November 10th. MobileTracker has also reported on this outage.
The few times that I've needed to talk to people at T-Mobile, I haven't had any problems.
I think AT&T's customer service issues and occasional service glitches with both carriers are forgivable. But because of the pricing ($20/month vs. $80/month), I'm using T-Mobile for my GPRS data. If AT&T Wireless offered unlimited high-speed (EDGE @ 70+kbps) data for under $50 a month, I'd switch. However, if T-Mobile offered the same, it would be a toss-up. Right now, it's down to the price for me.