Upstairs definitely. I like the side bins for
holding my stuff and for larger things, I'll usually sneak something into the closet near
the stairs. I agree with those here who have
said that they like upstairs because it's sort of "verboten territory" and you don't
have to worry about the kids tromping up and
down while you're trying to sleep.

Downstairs you have to worry about being stuck in front of the movie screen when you
just want to sleep or just plain not subjected to whatever's on.

Peeling the onion of my mind, I do recall years ago on a -100 going ORD->HNL, sneaking
up the stairs(spiral!) and hanging out for a while 'til the purser chased my brother and I out (We were like 11 or 12 at the time)
It was actually a lounge then w/ plastic molded tables and chairs....