SIT 'N' STROLL absolutely worth every penny. It is a car seat that turns into a stroller that turns into a flight seat and can be a rear facing seat and a booster seat as well. It is advertised in many airplane magazines. I loved it so much it became my primary car seat. I did use it with a Mighty Tight car seatbelt tightener to make sure that it felt as safe as possible. Here is the first web site to come up so at least you can picture what I am suggesting: http://www.strollerdepot.com/items/sit-n-stroll/
The best part is when they fall asleep in the car and you can turn it into a stroller and wheel them into your destination without waking them up, or having to sit in the car with a sleeping child. My other favorite was watching other parents struggling with a stroller and a car seat at the airport and knowing that my day was that much easier because of the Sit 'n' Stroll. Many people have stopped me at the airport and asked what it was and how to get one.
You'll especially like it in NYC because you'll have the stroller when walking down the street and when you need to catch a taxi you can easily convert it into a car seat and be on your way.
We loved this stroller/car seat and recommend it to lots of people who love it, especially travelers. Your little one is young enough that you'll get to use it often. I used it until we could not buckle my kids in any more and I pouted when they out grew it.