in a car its atleast soft and people dont walk by. ive slept in JFK at 3AM and felt like a bum. and since i have in fact been a bum i knew how much it sucked. lol.
I would choose to sleep in the airport but that is before I got the flu. I would much rather sleep in my own car which I know is clean and where I won't get sick.
If it's freezing outside or really hot I would prefer sleeping inside even if it was at the airport. They have bathrooms and food. Airport wins! I've met some wonderful people at the airport. You young people need to get off your cell phones or blackberries or whatever you have and talk to people!