It's really up to you.
I personally like the upper level for the better views and because to pass between cars (such as to go to the Dining Car or Sightseer Lounge) is only possible via the upper level. Others like the lower level for more privacy (no thru traffic), stepping out quickly for "smoke" breaks and the shower and most rest rooms are on the lower level. (There is also one rest room on the upper level.)
Some people try to avoid rooms 9 & 10, due to being next to the car end door. However, I have had rooms 7, 8, 9 and 10 many times, and have had no noise problems!
To me, any room on a train is good!