There's only one proper way to choose a cellular carrier. It's also very helpful if you learn how cellular systems work. One should always choose the carrier first, the one with the best coverage in your area.
The phone is only as good as the signal available to it. If you choose the phone first based on the availability or the price you'll usually be disappointed unless you get lucky.
I'm my area the strongest signal is provided by AT&T. Using GSM technology is a big advantage since they pass data & voice concurrently. Verizon (another good carrier) is crippled without this featured. It's CDMA technology can only pass voice or data, thereby the experience is limited.
Their claim of having the best network is nonsense, since it's a location based cellular system like all the rest. The cellular system is just plain expensive no matter how you cut it. The major reason Verizon spends millions of dollars per month advertising the network is that before they had Android to save them, there selection of phones was less than 1/3 of AT&T or anyone else.