I used to work for Consumer Cellular, and I don't recommend anything other than prepaid phones if you're getting them for a teen or as a gift. I used to spend a lot of time talking to people who got their kid a phone thinking it would get minimal use, only to find that their family member had managed to use literally thousands of text messages in a matter of days and there was no plan they could switch to that would make any significant difference in their bill which had quickly gone into the hundreds of dollars. If you're getting a phone for anyone other than yourself, go with a prepaid company that requires buying a block of minutes/texts/data in advance. That way, when they run out of minutes, the phone stops working until a refill is purchased rather than leaving you with a much larger bill than you ever thought possible.
As far as I know, Walmart doesn't have their own cell phone company. Even if they did, it would just be another reseller, like all the other prepaid and smaller cell phone companies. Consumer Cellular resells blocks of minutes they buy in bulk from AT&T. Tracfone, Net10, and Straight Talk are all the same company, which buys minutes from AT&T, Verizon, and Sprint. Virgin and Boost Mobile are both Sprint resellers. Resellers, however, get low priority on the networks, so if you live in an area where AT&T's network is very busy, you're not going to get very good service with a phone from an AT&T reseller because your calls will drop to make room for actual AT&T customers. I'd recommend trying to find people in your local social circle who use prepaid or smaller cell companies and ask them how their service is. The same company can have excellent service in one area and nearly non-existant service in another.