It's true that much of NYC is pretty flat, and if I lived elsewhere on it, I might be okay with a single-speed.

However, there's a 25% grade at the end of my 26-mile (round-trip) commute, and sometimes I tow my kids around in a trailer, so it really does depend on where you're going to ride and what you're going to do with the bike. Do you know where you're moving yet?