T-Mobile will have a higher network capacity. Won't get congested like att.
T-Mobile is using dual-carrier HSPA+, which permits more throughput PER CONNECTION, but actually the cost of overall capacity.
It IS unlikely that T-Mobile will get congested any sooner than AT&T's network, but that's more a virtue of T-Mobile having far fewer users on their networks.
And right now, and probably for some time to come, T-Mobile's major problem is LTE coverage. AT&T hasn't been very aggressive in deploying LTE, but T-Mobile is way, way behind.
T-Mobile will have less dropped calls.
There's no way to guarantee that. Dropped calls are a function of many factors, coverage among them, and T-Mobile doesn't have nearly the coverage footprint of the three larger carriers above it.
Where I live, for example, I'll get way more missed/dropped calls on T-Mobile than AT&T, and I'm not in the boondocks.
Att has better building penetration however newer technologies have improved T-Mobile penetration issues.
That's dependent on cellular density, and granted in some major cities in their core urban centers, T-Mobile has it. But it's not uniform.
Don't getme wrong: if the OP (or anyone else) lives/travels in the areas for the US where T-Mobile has decent coverage, then it's worth going there. But if you're not, you're going to be very frustrated.