1. Unlimited data at full speed? Or unlimited data at full speed up to 3GB (or some other low number), then 56kb for the rest of the billing period? There's a HUGE difference. (The "two-tiered unlimited" plans basically give you the full speed limit - what you can download 24/7 for a month at the low speed isn't as much as the 3GB you can download in a few minutes.)
2. Check the actual coverage where you use your phone. Find someone on TMobile, or make sure you get the "cancel within X days at no charge" option in writing, and test out every place you need coverage. (And I don't mean that if you work downtown, go downtown and check - sit at your desk and check. Dead spots can be inches across, and if you have to sit near the door every time you need to use your phone, it's not worth it. Check every room in your house. Sprint covers this town like a blanket. Walk into my daughter's house and there's a bit of signal if you hold the phone horizontally againse one window pane in one bedroom. But walk anywhere around the yard, patio, lawn - perfect signal. (And it's not the house - AT&T and Verizon are solid all over the house.)
"The SF Bay area" doesn't mean much. You could have solid signal, walk across the street and have none. In SF, in the business district, and along interstates, you probably have excellent coverage by all carriers. It's in your basement, deep inside a huge store out in the boondocks, places like that, that you have to check. If you're in the store, and they don't have the size whatever your wife wants, and you have to walk across the parking lot to call her to ask if you should buy 2 small ones - are you really getting a good deal?
I'm not saying that AT&T is good and TMobile is bad - you may find that TMobile covers the areas you need better than AT&T does. And their data plan may actually be unlimited at full speed (I haven't looked). But those are the things you need to check before making the choice.
And check out their service department. There is one locally for TMobile, right? I know there is for AT&T unless your nearest neighbors walk on all 4s and have tails and fangs. And I know they're good. (They actually apologized for having to replace one of my phones. You're giving me a new phone and you're sorry? I like that kind of service attitude [even if they weren't sorry], and I'm willing to pay more for it than "we'll have to send your phone out for repair - it can take from 2 to 6 weeks". I have spare hones - do you?)