Clearly, flight duration is a big factor. Additionally, to me, what's almost equally important is whether I know my seatmate(s). It would be much easier spending 6+ hours on a transcon in Y if I were sitting next to my family, friends, or colleagues, as opposed to sharing tight quarters with someone I don't know (and might not want to get to know). Sitting in F/J, particularly when I'm traveling solo, tends to be a more sure bet that I won't be bothered by any seat neighbors.