ATT is the winner at this point, in my opinion at least.
While yes Verizon currently has a bigger LTE footprint, the iPad now supports HSPA+. What this means is that with ATT you have 4G in their LTE coverage (73mbps) and when out of range you fall back to HSPA+ (21mbps).
With verizon you would have 4G in their LTE coverage(73mpbs) and fall back to 3G when out of range of LTE(3mbps).
I wish they would have put in radios for both carrier frequencies.
My iPad with the original unlimited data plan is on AT&T. AT&T has a large coverage area claimed for 4G in my area. I do not have any personal experience with the 4G, but I have always had good service from them. No brainer for me.
AT&T at the most 4-6mbps down, and a lot of times in buildings it doesnt work well. Verizon works everywhere, and the 3G is snappy even though speed tests show it slower than AT&T the web pages come up quicker.