I recently switched from Tmobile to AT&T (through straight talk and now Cricket). I have yet to find a place where AT&T's service isn't better than T-mobile's.
As for data speed, we're restricted to 8MB on AT&T, but that's plenty to do anything you would want to do on a phone. My GF only has 8MB at home, and we're still able to stream netflix, play playstation, and stream youtube all simultaneously without issue.
The other nice thing is that since AT&T throttles us to 8MB, we rarely see less than that. Whether I'm on LTE or HSPA+, I usually speed test at 7.5MB or higher. With Tmo, I usually tested around 12MB, but you never really know if you're going to get 3MB or 20 MB at any given time.
AND, if you hit your bandwidth cap, the same applies. We're throttled to 128k, but you actually get that speed, which is usable for web surfing, voice search, and usually even streaming music. On T-mobile, once I hit my cap, I couldn't do ANYTHING. Most speed tests clocked me below 56k if I was throttled. Simply trying to get gasprices.com (a simple site) to load made me want to punch babies.