Research GSM Carriers Carefully
One thing to be aware of is that you will need to have your existing T-Mobile phone unlocked so that you can use the SIM cards from these other companies. Fortunately, T-Mobile's practice has so far been to allow their phones to be unlocked, provided you've had the device for at least 90 days if you're under contract. If you have a T-Mobile device that's not under contract, they'll unlock it on request. I've done this with the T-Mobile phones I've had over the years, mainly so that I could use a foreign SIM card when I travel. In addition to the companies mentioned, there are smaller, regional GSM carriers located around the U.S., largely in rural areas. Most of these companies accept only local users because they don't want to have high roaming costs, but if you live in an area with such a regional service, these companies are worth checking out. One of them, Cincinnati Bell, covers a large urban and suburban area in three states, and offers the same services as any national carrier.
While the ultimate decision is up to you, there are several factors to consider. One is to check with the carrier to ensure that your 3G or 4G service will work with their system. You could find yourself with a 4G T-Mobile phone that won't do 4G because the carrier you're using roams on AT&T's service, and AT&T doesn't support it. Or you could find that they're not set up to bill for 4G, which could mean you can't use it. You'll need to ask to make sure.