There are numerous threads on this topic already, but generally you should always go for upper deck.
It's 1-1 seating and these will be going away soon. You get a smaller, quieter cabin (with 2 dedicated lavs up front), and almost zero foot traffic.
Only downside I can think of is that larger carry-ons most likely won't fit in the overhead bins (they are smaller on the upper deck to the curvature of the aircraft fuselage), but FAs are generally pretty good about stowing carry-ons in a closet by the galley.