Keep your 16, but go to a 46 (or maybe even a 47). As flat as NYC is, you'll still need some top end, so the very slight gear inch reduction you'll get with a 46 may get you the best of all worlds. Going to a 17 or 18 in the rear will make a much bigger difference, too big for the streets of NY, IMO. Check it out on a gear chart.

I have two fixes; one with a 46x16 that I ride around here on a moderately rolling course, and one with a 50x15 that I take when we go on vacation along the very flat Atlantic coastline so I can still get a great workout.