There was a thread discussing this on another board. We've had only three HAL cruises, two with upper main seating(8:00) and one with lower main seating(8:30). I think it is just a matter of personal preference, but we did prefer upper. On our cruise last month it seemed warmer (too warm) in the lower level, but of course we were not on the upper, so it could have been just as warm there.
the odam is running a test regarding dining room times -- if you are booked for either of the early times your dining time window is between 5:30 and 6:00 you must be in the d/r by 6---- you will be assigned a specific table--- no difference between upper and lower---
We are also on that sailing and I booked late dining upper. I'm reading now that the ship does open dining so don't know what that is all about. We like the upper because it's more quiet (no engine noise) and not so claustrophobic to us. Either way can't wait for this trip.