If your area has AT&T LTE, and you don't plan to leave the area, I think AT&T will be the significantly faster service. They have far fewer devices on the network, and likely will at least throughout this year. If you do leave, you should have pretty good HSPA+ coverage. It's not LTE, but it's still capable of doing 7mbps in many areas. Maybe it will be even higher than that. I don't know.
I chose Verizon because Verizon has LTE coverage all around the northeast, where I am primarily, and AT&T only has coverage in Boston and NYC. I was in Vermont this past weekend skiing, and 4G LTE was available from Verizon. I am not sure if AT&T will even have LTE this year in my area.