You get less muzzle rise from the bottom bbl as it is more inline with your shoulder. The recoil of the top bbl is high on your shoulder and it is above the center of gravity of the gun - so you will get more muzzle rise.
There is no correct answer. Try both and see which works better for you. When I shot an over and under for singles I used the bottom barrel. So when I shot doubles I was always more successful using the bottom barrel for the second more difficult shot because the p.o.i. was second nature.
Many competition doubles barrel sets are regulated so that the bottom barrel shoots the highest POI. For that reason, most shooters go with the bottom barrel first, but you need to pattern the gun to see if there are any "surprises". You never know, y'know?
Do what Dave S. says about shooting both bbls. at 13yds. to check POI of each. I had a DT-10 that I had to shoot the top first because it shot considerably higher than the bottom. Not all like O/U guns necessarily shoot to the same POI.