It all depends on how the aircraft is configured and therefore pretty much which airline. If FC is in the nose section, I prefer UD to main deck. If it's BC in the nose, I prefer main deck, especially row 1. No one goes up to row 1 except to get to their seats or to the closet (not an issue for most of the flight). Really nice and quiet up there. On the upper deck on 747-400s, I hate sitting in an aisle seat as I can feel the vibrations of heavy walkers when they walk past my seat.

As for the privacy and exclusiveness. UD on the old 747s - the 100s, 200s and SPs - will feel more exclusive than any 747-400 and -300 ever will.

So, I like UD in WBC on NW 742s (old FC seats vs. BC seats on the main deck). On NW 744s, I prefer main deck especially row 1.

Similarly on certain 744s operated by O8, the nose section is preferred over the UD: nose section may have old NH FC seats while the UD has old NH BC seats with really cramp BC seats. Doesn't matter on the airline's old SQ 744s.