It really comes down to what works best where you plan to be...
I think this is key! Nothing else matters if you don't have signal where you live/work/need to be.
I live (and have my home office) in a town that basically prohibits cell towers. Lately they have relented some if the tower is installed on town property (so the town gets the revenue), but refuse to permit them on privately owned property.
That said, I've had most of the big dogs . . . started with Cellular One, then Verizon, AT&T and finally Sprint. Had absolutely no signal at my home from anyone but Sprint and after Sprint sued the town in USDC, a tower was built close enough to me to make a huge difference.
Financially, nobody except Sprint ever offered me any money off upgrades, I paid their full retail each time I upgraded prior to Sprint. Sprint also gives us 23% off the basic service . . . available for anyone in the medical business/working for a hospital. I never was aware of any similar discounts from any of the other big dogs.
It was years ago but when visiting with a friend in Sunapee, NH neither AT&T or Sprint had any signal. Franconia Notch area was mostly dead on Sprint a couple of years ago. A number of years ago anywhere outside of Downtown North Conway (in that area) was dead on AT&T. No problems with Sprint around Concord, Manchester or Nashua NH areas. Sprint also was solid in the Miami-Boynton Beach FL area. In this area I've noticed an improvement in signal strength and more LTE with Sprint over the past 6 months or so.
I've now been with Sprint since 2003 and have unlimited data and unlimited minutes. As long as I don't have to deal with their customer service, I am happy with their service. When I upgrade my phones every ~2 years I do it at Sam's Club by waiting for one of their special sales.
I will admit that Verizon has the absolute best coverage.