Keep your seat. F on a 757 basically sucks to begin with, and this is a red-eye, so after about the first hour, lights out, forget about any great service. Sitting in row 9 gives you tons of room to strech out, and since you will probably sleep, it would be a waste of stickers in my opinion.
From my experience F gets full on these flights, so if you're thinking about having 2 seats, I would say the chance of that is slim to none.