Safest deck on multi deck airplanes It is a well known statistic that rear seats has higher survivability rates than front seats due to generally lower impact forces. 

Double decks airplanes are not so common so statistics would probably be unreliable, especially since Boeing's only have a forward double deck. 

Has any survivability study been made on that kind of aircraft, that include upper vs lower deck ?

I'm mostly interested in the a380 but other design are welcome as well.
Very difficult to say as we don't have enough real-world examples.

Vertical impact forces would be lower on the upper deck as the lower deck acts something like a crumple zone and absorbs some of the acceleration.
That said, an evacuation from the upper deck can result in more injuries due to the longer, steeper, slide that's needed.