By the time my contract is up, LTE iPhones will probably be available on both AT&T and Verizon. At that point I will re-evaluate and decide whether to switch or not. SO MUCH can change so quickly in the industry.
I have about 21 months left on AT&T, so it won't be anytime soon. I'm fairly happy with AT&T service in my home area (South FL), it's only been a little dicey in Manhattan, and parts of downtown Chicago.