Best GSM carrier for a trip

I am going to the US next week and am picking up some sims tomorrow. I am thinking of going walmart $30 for a month with my phones but was just wondering if theres anything better. I have a Galaxy Nexus and an iPhone 4 both unlocked that I will be using and the iPhone does not need data.

Also who has a better connection in Orlando (Disney world) and Washington DC AT&T or T-mobile