In the US, AT&T has by far a larger 4G LTE coverage footprint than T-Mobile, but T-Mobile is generally the faster network. The US market is very different than ours because spectrum is sold by market, so in one area T-Mobile might have 20MHz of spectrum and AT&T might have 15MHz and in another the other way round. AT&T is generally a solid network, not the fastest not the slowest speeds but always useable. T Mobile is more of a risk for many people. You have the advantage of both when roaming.
There are two other main networks (there are lots and lots of regional networks and affiliates) Verizon and Sprint, you can't roam on either of them because they're CDMA based, although they're using LTE even though Sprint dabbled in WiMax. Verizon has the best coverage footprint of any network, indeed their LTE rollout is fully complete and generally has very good speeds especially on their XLTE (20MHz wide band rollout) network. Sprint is behind quite a bit because they messed up their rollout at first, they used a 5MHz bandwidth carrier as their primary frequency for a time (à la Three) and it's really comeback to bite them, now they're rolling out on three frequencies using 8T8R antennas on every mast, FDD800MHz at 5MHz bearer, FDD1900MHz at 5MHz (10MHz in very very few markets like one or two at most) and TDD2500MHz of which they have about 120MHz of bandwidth of which they're rolling out 40MHz in CA in their most used markets.
Long post but I thought I'd share.