Soup is ready to get his first smartphone. Leo says the number one thing to decide is which carrier he wants. Then choose the phone from there. He'll want the carrier that offers the best coverage in his area. Service-wise, they're all horrible in some way. But coverage is better for some than others.
T-Mobile is the best of the four and they're very aggressive in pricing. AT&T is the “fastest” for data. Verizon has the most coverage but is slower and he can't do calling and data at the same time (though that's changing). Forget Sprint, they're in trouble for adding unauthorized charges to users accounts. T-Mobile also has unlimited text and data with 100 minutes of calling for $30. He'll have to hunt for it, but it's available.
Then there are companies that resell services from AT&T and Sprint, called MVNOs. But the service isn't all that good. Some services will let him to try before he buys, basically returning the phone and cancelling the service with no penalty if it doesn't work well for him. But T-Mobile is by and large the best carrier for Leo.