Upper deck all the way baby!!!!!
1 - it's more private
2 - how many other jets can you fly with stairs and an upper deck - only the A380
3 - less foot traffic means better sleeping and quieter all around
4 - 2 bathrooms easily accessible
5 - more attentive service IMHO
6 - more space ( maybe debatable ) but love those side storage spaces but the new seats don't have this

I preferred the very last row if it were new seats on the upper deck or the very first row.

If old seats I like the very last row of the most forward section just before the emergency exit door.
Enjoy CX is like no other