From my experience (I've worked for Sprint and AT&T) the breakdown is this.
AT&T has the most expensive single plans, but incredibly good savings on family plans by comparison and the second best coverage out there. But their non iPhones are largely boring in both color and what they can exactly do. The AT&T machine invests heavily into the success of the iPhone, so that's pretty much their star player.
Verizon as far as my own independent research shows has the best coverage out of any provider period. They have horrendously high pricing for prepaid and family plans. And although their cell phones have a little more "umph" in terms of features and marketability, they also suck. lol. There is a reason they are on the Chocolate 3, The Envy 2 ect. ect. They invest heavily into the cool looking LG phones. But frankly, LG sucks as a manufacturer.
Sprint has the coolest looking phones and by far the cheapest price plans across the board. But they also by far have the crappiest coverage overall. Sure they will work great in just about every major city, but beyond that you're buying a really flashy looking paper weight. Also a heavy reliance on LG, like Verizon, makes most of their phones make poor stability.
T-Mobile is a dark horse, as I know the least about this service. Probably due to poor marketing when it comes to getting their name out there. I know they've got an incredible fave 5 or fave 10 plan. And they have something called Flex Pay which allows you to basically have a prepaid billing with post paid coverage and phones. However, from what I've seen their phones are also largely boring like AT&T, their customer service apparently sucks (word of mouth here) and their rate plans are very middle of the road.
If I haven't mentioned any other cell phone providers, they are probably even smaller than T-Mobile and thus probably not worth looking into.
Personally, I have Virgin Mobile, a prepaid service. Because it's dirt cheap. I have a Pantech phone, have had it for over a year, and its still going strong. I had Verizon, and never had a complaint about their service. I had a Motorola with them, which was very reliable.
So when searching for a cell phone service, it really depends on what you're looking for. No matter who you go with I probably recommend getting a Motorola or a Samsung device, as they seem to be the funnest/most reliable.
Of course being that I work for AT&T currently you should all buy those phones. lol
