1) where are you planning to use the phone? This is important if you travel or regularly spend time in a place like a cabin in a location with limited cel phone coverage;
2) which carriers serve the locations you spend time at, or have good agreements with foreign/out of state carriers where you travel to?;
3) do the carriers that serve the areas you spend time at have plans - either prepaid or locked in month to month charges - that meet your needs and are at a price you like?;
4) which phones work with the communication protocols/bands used by the carriers you like in the locations you will use them?;
5) does the carrier whose plan you like offer a phone that works in the areas you will use it that has the features and physical configuration you want, at a price you want to pay? Sometimes it is worthwhile locking yourself in to a contract for a couple of years. Sometimes it makes sense to buy a separate unlocked phone and signing up for a "no commitment" or "one year commitment" plan.
See - it's simple!
It obviously helps if you live and spend time in an area where there is lots of coverage.