I seriously considered the Verizon one, since LTE is available locally and it is unknown when LTE will be here on ATT.
But I didn't for the following reasons (kind of conflicting at any one time):
At least the new iPad will support HPSA+, which is available here, so it should be faster than my original iPad 3G.
I have the unlimited plan still in force. So I can have unlimited with the new iPad.
There is the possibility that ATT may, someday, offer a way to share data with both the iPad and my iPhone. If so, I could save money. Obviously, conflicts with the unlimited plan. I would have to give it up if it became available and I switched.
On the flip side, I learned today that Verizon will support a hotspot with no additional cost. I doubt that that would be available on my iPad with my current plan. Of course, if I need it, I can turn it on my iPhone for the month I need it. I haven't needed it yet since it was offered officially on my iPhone.