I opt for lower. Row 12 is always my favorite, just in the middle. Interestingly, whether the aircraft is 744, 74J, 333, 34C, 346, row 12 happens to be my favorite.

Reason as others pointed out. It's simply got higher ceiling and less cramped. Also, on a not so full flight if you choose asile of the middle seat, it's almost definitely certain that you will have empty seat next to you. On upper deck, is very unlikely.

Also, you get to board off the plane very quickly.