I personally have T-mobile, and love it. Great price, blazing speeds, and great coverage where I am at (small city, lots of rural area...tmobile has improved drastically lately). I even got full on LTE at a recent college football game, surrounded by concrete walls, with 80,000 people. No one else around me (including my Dad, AT&T) did. d
I have had all four carriers in recent history and I find T-Mobile to be the best for price, promotions, and customer service.
That being said....you have unlimited everything for about the same price as T-mobile, on AT&T? That a pretty good deal.
Answer these questions:
1. How important is customer service (T-mobile wins, IMO)?
2. How important is promotions (discounted iPhone at launch, etc....T-Mobile wins)?
3. Is T-mobile rock solid where you live, all the time, everywhere?
4. Is it reliable else where, in the case that you travel often, or at least often enough?
5. Assuming the answer to 4 & 5 are yes, are you staying there for the foreseeable future? If there is change you get moved to middle of now-where Montana...you t-mobile might not be your best bet, yet.
6. Finally, is the speed difference that big? If you are undecided still, this might be the deciding factor.
I love T-mobile, but I am not blind. AT&T has coverage in places T-mobile doesn't. I think T-mobile will continue to improve, but that doesn't help someone, like I said, in the middle of Montana.
**EDIT: Yeah, the test drive thing. Great idea. Look at the coverage map too. Areas where my family lives used to be completely uncovered, with T-mobile, and now its all high speed LTE.