isnt it all just roaming
1. Price/ Value
2. Phone Selection
3. Home area coverage
4. Customer Service Support
In that order. If you live in a big metro like chicago the home coverage area will be about the same. In other smaller cities certain companies are going to perform better due to their towers being primary on the prl roam list and other carriers wont have this priority listing. Customer service is not big for everyone but some its the world. Overall you are correct the biggest factors are price/value and phones.