Seriously, DO NOT get AT&T in NYC. Yes, some people will inevitably come on here and state how great their speeds are based on a speed test. But, the fact is, AT&T is terrible in Manhattan and speed tests don't reflect real world usage scenarios IMO.
I was a happy AT&T customer for 5 years in Colorado and then moved to Manhattan 6 months ago and was been anxiously awaiting the release of the next iPhone every day during that 6 month period so that I could jump ship and switch to Verizon. I had 5 bars with AT&T in my apartment & pretty much throughout my neighborhood, but calls would frequently drop or not even go through when attempting to dial. Data was unusably slow a lot of the time. AT&T replaced my SIM card which yielded no improvement. Apple replaced my iPhone, again, no improvement. AT&T's network is simply way too congested here. Despite having a full signal, the towers are overloaded and voice and data bandwidth is reduced for everyone.
I haven't had Verizon long enough to offer conclusive results, but I strongly believe Verizon has the best network in NYC based on numerous posts I've read and comments I've heard from others around town over the past few months. So far it's definitely an improvement over AT&T. I haven't dropped a call yet in 4 days and people's voices don't break up like they did with AT&T (despite having a full signal). I almost always had to call people back with my home phone whenever I got a call on my cell in order for the call quality to be good (I work from home, so a land line is a requirement for my job).
DO NOT go with AT&T in NYC.