AT&T is second to Verizon in terms of customer base, but the company hasn't let that stop it from aggressively building out its 4G LTE network. AT&T boasts better reliability than the competition, with high scores from independent testers for call quality and Internet speed.
While VoLTE will level the voice/data playing field in the future, AT&T's use of the GSM networking standard makes simultaneous voice/data communication the way things have long worked on their network, so you don't need to worry about the switch getting flipped in the future — it works now, just as it always has.