For me, AT&T. I have an iPad 1 and I want to keep my unlimited data plan so I can't switch. Also, at work we have horrible Verizon coverage so AT&T is best for me.
Most of what I've read indicates that while VZ's voice network is superior to AT&T in NYC (which isn't hard), AT&T's 3G data speeds are much better, though perhaps not as reliable.
You would need to get replies from every people single state in order to paint a proper picture of nationwide speeds. Pretty sure that ain't happening. All you'll get is a smattering of anecdotal experiences. For some, AT&T is better. For some, VZ.