C.M.
Take the free seat. First of all it is impossible to lug the car seat through the air port. And is it really necessary? I mean if the plane goes down, will the car seat save her? when my girl was your daughter's age we did a similar trip. You will find that the airport staff will accomadte you with a small child over others if they have to move people's seats around. On one of the flights we even ended up in first class for no extra charge. Bring a pacifier along if she uses one to help her with the ears popping, or a bottle. We even had our pediatrician give us advice. He gave us a dosage for draminine, and that worked niceley. You will probably find that she will want to sit on your lap the whole time anyhow. Don't forget to bring a carry on bag full of her favorite toys, blankets, a change of clothing, crayons, ect,, just in case. But I would check the car seat with the luggage....way less hassle. Also 600 is a lot of extra spending money that could better be used for Christmas or Gas money ect.... Or take the extra money with you. There are always more open seats in first class. Sometimes, at the last minute the airline will allow you to update your ticket to first class for 50-75 dollars and that way she'll have her extra seat for way less money. Good luck and have a great vacation!