Window. I remember one late night when I had to ride the bus for a few hours and when I woke up, I was all cuddled up on the shoulder of the man next to me. I was embarrassed and apologized profusely. He said it was okay and to go ahead. I very carefully turned in my seat so it would be unlikely to happen again!

Anyway, like everyone else has said, window allows you to lean against the wall and I'm usually sleepy when I fly because of having to be at the airport early or being tired from the festivities wherever I was.