WiFi

I can't speak to AT&T, but I have unlimited data on Verizon, and it works pretty well. Depending on your destinations, the signal may be weak or non-existent, but assuming that you can get the signal, you should be good. Also be aware that phone apps don't always work as well as a computer when trying to do things like streaming video, but that's mostly an issue with poor app design, not the service itself.

I'm also thinking of buying an adapter to send my phone's signal to the TV, but I haven't tried it yet.