Last year, AT&T and T-Mobile entered a new mode of competition by acquiring some old networks. Both companies picked up some spectrum, some customers, but more importantly, a way to compete in the low-cost arena without diluting their main brand.
Picking up a Cricket Wireless phone gives you a slick way to get "AT&T Light". It's AT&T with a, "We'll do what it takes to compete" way to doing business. The same is true with MetroPCS which gives T-Mobile a chance to push the low end without cheapening the T-Mobile name. Both services have much to offer including a large selection of phones and competitive plans. They both even have a Mexico roaming package.
As I drove across eastern New Mexico at Christmas, I noticed that my Straight Talk phone never switched away from "AT&T" instead of the old "Roaming" label it displayed last time (which happened to be GSM supplied by Verizon). In this case, AT&T has taken over their recently-purchased Plateau Wireless network, eliminating my last objection to going with a single-network product like Cricket. Of course I won't want to travel across Nebraska until AT&T makes a presence there.
As we dissect each of carrier's offers, the only difference we can find is that Cricket's Mexico add-on gives you 100 minutes for $10 versus MetroPCS's 200 minutes for the same fee. Both are a great deal since going to Mexico either required a pricey roaming plan or the need to pick up a GoPhone phone or SIM. Look for reasonable Mexico Data roaming charges in the near future when AT&T acquires their own network in Mexico, which should also give Cricket a leg up. Chirp-chirp.