Sorry if this is well-tread ground. We are traveling by air this summer. DD (almost 4) has her own seat on the plane. Is it preferable to bring the carseat onto the plane and fasten it into her seat, or gate check it? I'm guessing the former, in which case my question is, will a Britax Marathon actually fit in a standard plane seat, and, how much of a hassle is it to install?