I prefer downstairs because you have a place to put your bags. The problem with upstairs is overhead bins are smaller, the side bins only allow a briefcase to fit and most business class seats have no space to put anything underneath them.

As an aside. Why is it that Korean Airlines always puts me upstairs been I have confirmed business class seats downstairs? They say that business class passenger perfer upstairs but most heavy duty frequent flyers I have met prefer donwstairs.