K.B.
First of all, there would be an oxygen mask for your daughter if she's on your lap -- there are at least 4 per a row of 3. So don't get the extra seat for that reason. One thing I have done is to lug the car seat with me out to the gate in case there are extra seats, then they cn move you guys around and get you near an open one to put the car seat and child in. If not, you can gate check the seat and have it waiting for you when you arrive. As for her comfort -- is she the type who will sleep happily in a car seat? Does she sleep on long car rides? For everyone's comfort, if she will want to or need to sleep, having the car seat to have her snug in is great, but the cost can be a lot. Gotta weight those things.