With acoustics like that i would recommend you try both and decide from there. Facing toward the seats might "bounce" more off of the seats, although most people that put subs in trucks aim them up or downfire them. Unless you are building a wall anyway.
I guess that it is a preference type of thing. I would have to agree with taylor on the fact that when aimed forward at the drive it doesnt tend to have as much SQ. Is it hard for you to try out both to see which one is best??