Two weeks ago I moved my wife’s iPhone 4S from AT&T to StraightTalk’s bring your own device service. StraightTalk, a TracFone company, re-sells access to T-Mo or AT&T wireless networks. As such you get the same signal quality and speed that you’re used to.
I haven’t tried using StraightTalk with an LTE phone, so I will not comment on that. However on HSPA+ phones (My wife’s iPhone, My cousin’s SII) it performs identical to AT&T in my testing of data network performance. The sound quality and texting seems about the same as well. There really is no observable difference, which makes sense since it is the same network. My advice is to do the math and see what makes sense. For example here’s what the math told me: