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Generally speaking, AT&T has definitely improved in NYC. I have an AT&T iPhone 4 and picked up a verizon model yesterday because I have a few "problem areas" where I do repeatedly get phone calls dropped and low signal. Low and behold, the verizon model barely got a signal in the same area, and kept switching between 1xRTT and 3G. And through and through, I find that AT&T 3G is significantly faster than Verizon when out and about, which has been reported everywhere.
Places I frequent for comparison:
UWS, hell's kitchen/central park south area, and Washington Heights. Also was on staten island yesterday and didn't discern a difference in signal quality near me and definitely saw the same speed difference.