AT&T has recently made major improvements to their network, so I am not going to say they are not as good as Verizon, I also won't say they ARE as good or better, I simply will not say since I have not put 'em up side by side.
I do know this: There are really only a few PROVIDERS, there are many cellular companies but most of them re-sell one of a very few PROVIDERS T-Mobile, AT&T, Verizon and Sprint are the 4 providers I know of.
US-Cellular uses CDMA 2000 technology, Same as Verizon, Which makes me thing that the coverage is likely right close to the same either directly or with roam agreements.. That said Wikipedia says that US-Cellular does operate their own network, not a reseller like so many are.