The bottom line is that it matters where you are, AT&T and Verizon included, though they have the largest coverage areas. Even with the same city, coverage quality can vary dramatically, due to both signal available and network saturation.
Given the lack of price difference in many usage scenarios and a consistent set of popular devices supported, the right choice for your carrier more and more comes down to where you use the service.
Clearly, the competition among four carriers has done nothing to spur meaningful innovation, whether in technology or in business approaches. In fact, it has made the carriers more the same, while also raising prices. For example, Verizon's Everything plans last summer actually raised total spend for customers, yet customers did not flee. That sent a strong signal to the other three that there was room to charge more -- and they have.
You have to wonder why the United States bothers with having four cellular carriers, considering the similarity between their offerings. It would frankly make more sense to unify the wireless spectrum and have all the carriers use it based on demand. This approach to using the public airwaves would get rid of the silly lock-in and overcome the disparate service quality from region to region as smartphones and tablets would no longer be bound to one network.
That notion has a snowball's chance in Hades of occurring. If there's any comfort from today's cellular plan quadropoly, it's that you can focus on what matters most -- service quality -- and worry about pricing only if your area has at least two strong providers.
The table below shows the current state of data plan pricing from the national carriers.