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NYC: Both seem to work fine until you get upstate, in which case Verizon dominates.
As a resident of Upstate NY, I've got to disagree. Both AT&T and Verizon get excellent service in the area, with one coming out slightly ahead depending on the town.
In my own, AT&T's service trumps Verizon hands down. Hence, I'm switching to AT&T upon announcement of the new iPhone.