old guys with bad backs get the bottom bunks aft, where it's calmer but noisier. young guys get top bunks forward where it's quieter but you get beat up more. remember, we may be running four nights out of five. everyone else get's something in between. the main consideration is safety while climbing in and out of the top bunks. there is no ladder.
My back is good, my legs not so good. I can sleep in a smaller hole if there are size differences. Bouncing or engine noise does not bother me.