I love all these T-Mobile bashing threads because everything boils down to what happens when you're out of a city. I travel for work and T-Mobile works fine. Why? I'm either in a major metropolitan area or I'm in the air. I never drive out into the country, and if i do, I don't mind the disruption of LTE service because it will always be there when I get back to a metro.
The only place this gets annoying is if you are trying to use Google Maps, but price it out. T-Mobile w/ Unlimited Data + a Tom Tom or Verizon with a cap.
Personally, I'll take T-Mobile because you can abuse your unlimited data (I download movies over LTE all the time and can set up a hot spot that beats whatever hotel wifi will have). Yes, YMMV. You need to really look at where you're going to be. Flying into NYC or Boston for the week for work? T-Mobile. Driving around in the middle of nowhere and using GPS? Don't T-Mobile
(p.s. If you buy a T-mobile iphone they'll lock the mobile hotspot to only 5GB / month. You may be best off with a 'droid device if that's going to be your primary use because that won't be locked -- although you can download unlimited data to your phone and transfer it over e.g. with Airplay)