voice calls are less tolerant to a bad signal. Data, whether internet or text can handle fluctuations in signal strength and quality. Basically the data connections might be 'dropping' for a split-second and you don't even notice. If you lose signal while on a voice call, even for a second, then the call terminates.
To OP;
By the way, I have used ATT 3g service voice, iPhone, and laptop card at wdw and never had a problem. Maybe not as fast as your home wifi, but defenitely sufficient.
Honestly, I think if you stay with he big four Verizon, AT&T, Tmo, or Sprint, you should be ok.
Also, I assume you don't mean "real wifi" like hotspots all over wdw. If that is the case don't expect much.
Per other thread: Yes, I am a heavy lurker trying to get some more facetime....