From when I was on the Triumph, I thought they tended to assign passengers from the front half of the ship to the center London dining room, and passengers from the back half of the ship to the rear Paris dining room.
Of course, being part of a common group that wants to dine together, and that group having cabins spread all over the ship, might screw up any normal way of assigning.
I don't think you can request a specific dining room.
I agree that the Paris dining room, especially the upper Paris dinging room, is the best. 