I got rid of ATT and use TelCel Sin Frontera (Amigo prepay). It worked great all summer in the USA while traveling for 2 months. In places, there was no coverage. TelCel uses the T-Mobil network. And, data was sometimes slow. As a backup, I've always had an ATT hotspot which I use in tight situations. I can then use Skype to make phone calls if the T-Mobil net is unavailable. So, in addition to paying about $300 pesos/month for my TelCel plan, I pay $50 per month to ATT for data for the one or two months I am in the USA.
You live in SC. That's a different scenario.