I sell cell phones on all the carriers, and I am just going to tell you be very VERY careful with tmobile.
I've heard everything about this company and have seen it. I move at least a dozen people off tmobile to one of the other three, I'd be weary of sprint too...anyway here's a list of common complaints about tmobile that I hear down here daily:
Dropped calls, slow 4g, they told me 99 for a phone and now they're charging me another 20 dollars a month for 20 months (499 for the phone), horrible customer service, after they make the sale they don't care, they will find any reason they can to renew contracts whenever you call them, they won't give me my account number, they wouldn't honor an insurance claim, when I call to try to upgrade they convince me that the phone I want isn't what I need and give me the phone they want....the list goes on and on....
For the other three here's my 2 cents....
Verizon - largest lte network (good if you travel or live in a rural area), high end smart phones with a mix of CDMA and gsm, horrible customer service reps (even to those of us who sell it for them), less work discounts and slightly more expensive plans.
AT&T - by far the best gsm network in the us (i also have them), my iPhone 5 screams speed test data usually gets me 27-37 megabits. Customer service is the best I've found out of the big ones although it could use some polishing, high end newly released phones, plans comparable to Verizon, however the company offers more discounts for more people. Plus with AT&T you can talk and surf at the same time (although Verizon is starting get some phones that do that), if you're going to travel overseas as well this will be your best choice.
Sprint - avoid like the Black Plague! Their infrastructure is about 3 years behind and they have started charging EVERY customer a $10 premium data fee to help facilitate the expansion of their network. Here's the catch though that fee will be a perm fee because they will always be trying to expand. Right now they are a glorified 3G network with very few regions with 4g coverage, all the wimax coverage is coming down and I can't tell you how many people drop sprint like a bad habit like tmobile, actually I had a guy bite an ETF just to transfer to a different carrier yesterday.
I hope this helps, if there's any questions I can answer for you let me know. Also be sure to check out your local stores such as RadioShack, BestBuy Mobile, etc... Avoid Walmart Mobile they don't have any idea as to what they're doing and avoid the stamped places that say "authorized reseller" because 99% of those are commission driven and will used car sales guy you into what will make them the most money.