If I were in your position, I wouldn't even consider getting a single speed. These days, derailleurs and shifters are quite reliable and don't add all that much to the cost of the bike. Also, if you're going to be riding in traffic (and in NYC there's no way not to ride in traffic), you're going to want to be able to accelerate quickly and cruise at speeds that allow you to interact with cars. If you're not a really strong rider, you will really want gears for that. If you do some off-road riding at some point, you will also probably want gears.