I might be wrong, just my personal opinion, so please don't get nasty:

I always find the F/As at upper deck are nicer, more cautious and friendly than lower deck, it is probably because a lot of experienced travellers and frequent flyers (1K and GS) like to seat on upper deck. It is more quiet up there since you don't hear much noise from the engines or baby crying. I also notice seats at upper deck always get reserved and packed up first before lower deck.

Passengers with lower MP status and wait for upgrade usually don't have their upgrade cleared till the last 20 0r 30 minute before departure. By the time when their upgrade are cleared, all the seats at upper deck have already taken, so they have no choice but have to seat at lower deck. In addition, I find lower deck is more family oriented since it is not so convenience to carry their babies and baby strollers all the way to upper deck.

In my last flight from HKG to LAX during boarding, the lady sat right next to me looked very upset when she approached the seat. She immediately complained to the F/A about the seat she was assigned (she had 6G and I had 6H window, the first row of C behind F class). I guess her seat was a kind of close to the gallery and lavatory, so that was why she wanted to switch seat. The F/A asked her to wait for a while, then minutes later the GA came in the plane and talked to her in a very angry tone, "You know you are UA employee, you are already lucky enough to have a C class seat, why do you still complain about the seat you were assigned? The flight is very full, I will not move you". I was a kind of shocked by the way the GA talked to the lady in front of so many passengers around. Besides, during the journey, the lady demanded a lot from the F/As by kept asking F/As to bring her water, wine, more coffee, more desert, extra blanket ...... this and that, like never end. After this incidence, I don't think I want to seat at lower deck again even though I know it is an individual incidence.