Coverage is key, you don't want to be paying for a service you can't use so I'd make sure whichever you pick you are covered. I was told all Verizon Corporate stores are in a 4G area and while I don't know if that's true for other carriers, if it is, it could be a factor and you could choose a carrier that is nearby. Other factors are lifestyle, do you travel a lot? Have you purchased a home or do you rent; will you be moving soon? etc. etc.

I personally have had T-Mobile and while customer service was always there, service coverage was not; I'd often stand in a corner to make a phone call. I however was much younger and living off boxed meals and my profession didn't require me to need spot on service, where in now a lot of employers will text you for various reasons.

I don't think any carrier is perfect, but you just have to pick one that is right for you. Just keep in mind the longer you're with a carrier the greater chance when prices go up, yours won't if you're on a good grandfathered plan.