I've flown with Suzannah when she was 11 months, 19 months, and just shy of three years. 19 months was by far the hardest. We brought her carseat on the plane, but she did NOT want to sit in it and wound up kicking the seat in front of her a lot. But we really did appreciate HAVING the extra seat, because she was so wiggly and busy and, honestly, upset for much of the time (she had a cold).

Last spring she wound up sleeping on me for most of the flight, which was nice, although we HAD to buy her a seat by then...still, the space is nice. I'd recommend buying it if you can swing it.

Also, checking the carseat is a piece of cake -- I'd do that.