I prefer upper deck. Here's why.

a)
On the main deck, you typically have 8 seats across over 2 or 4 rows per cabin section.

The upper deck however has a configuration with 4 seats across over 5 rows which gives a "different" sense of space. In the upper deck, you do not have the middle seats which I think leads to the real sense of claustrophobia in a cabin.

b)
On the main deck, the walls on each side of the window seats are vertical and close to these seats meaning there isn't really any "space" in between.

On the other hand, the upper deck window seats are next to a slanted ceiling with storage bins on the side and a lot more space above these bins keeping you away from the fuselage and the windows. This results in a much more airy atmosphere.

Some say the upper deck can get a bit hot, I think it's fine and generally the seat next to the exit row is a little cooler (and one of the least claustrophobic seats you can get on the plane). The passengers on the upper deck also seem to be the most experienced flyers, the kind that have done it many times before and just want to be left alone, resulting in what I deem to be a much more quieter cabin.

Give it a try, only real way for you to know for sure if you'll prefer the UD or main deck Change is good!