Coverage / signal reliability varies by specific location. Bandwidth during peak times also varies, load increases then they up backhaul bandwith and it gets better for a while.
Full disclosure: I work for one of the four big carriers. But this isn't a commercial for my unnamed employer, it's just a description of why they are different.
There is a fundamental strategy difference between AT&T/Verizon and Sprint/T-Mobile. The key, of course, is money. And as a customer, you do get what you pay for.
Did you know that in the US, almost 70% of the population lives in 3% of the landmass? (That sounds shocking until you think about Alaska, Montana, West Texas, Nevada and Wyoming.) It doesn't take (comparatively) that many towers to cover the 70%. But it takes a disproportionately higher number more to cover the next 10% of the population. And the next 10% after that take almost half again the number of towers. The expense gets higher and higher as you try to reach 99% of the population (which is contained in roughly 70% of the land area of the US).
If you have the money to buy the spectrum and build the towers, you can choose to cover as many people as possible (and the side benefit is that you provide better coverage for people who travel a lot, especially to rural areas). If you have the money, you can also spend the billions on spectrum needed for the capacity to support users in dense areas and the backhaul to go with it. AT&T and Verizon, because they have the big subscriber/revenue bases and the cost advantages of legacy ILEC backhaul facilities in collectively more than half the states, choose that path. But it all costs money to do that, and you as a consumer pay more for the coverage quality.
Sprint and T-Mobile don't have the big piles of money or the huge subscriber bases. The good news for them: like I said, it costs a lot less money in tower building to cover 70% of the US population, and if you have fewer subscribers then you don't have to shell out as much on spectrum and backhaul. They have chosen (probably wisely, given their bank accounts) to go for the low hanging fruit, which costs less money and they can price their service more aggressively because they aren't trying to spend the money to cover everybody. So their strategy works well for most people, although if it works TOO well, then they have to start shelling out money that they don't have for more spectrum. (Sprint already has more spectrum than they know what to do with, but most of it is high-band ex-Clearwire WiMax spectrum that is almost useless in dense urban areas with lots of buildings to penetrate.)
So the bottom line is:
- Live in an urban area and spend most of your time there? T-Mobile or Sprint are likely to meet your needs.
- Live in a suburban/rural area, travel much and/or want to make sure you've got connectivity wherever you go? Verizon or AT&T are probably a better choice.