Put me in the lower deck camp. The upper deck has a more cramped feeling to me. Plus, I usually travel with a (very legal sized) rollerboard, so I don;t like hoofing it upstairs and sticking it in the rear cubbyhole. Although some have esposed the benefits of the storage space on the side, it's useless if you are in aisle seat. I also find that air friction noise is higher upstairs. Finally, I have really never had a problem with lav lines on the main deck - most carriers have 4 lavs below, and at least one of them is typically very roomy.

All things considered, there isn't that much difference. If you have no larger carry-on, try the UD.