I'm sure you know that it's ideal to have both dark, but if I was to pick one out of the two in a non-dedicated room, then I rather have dark walls, and light ceiling as it's more aesthetically pleasing than the other way around. I think a lot of it will also depend on how close is the screen mounted near the ceiling, if it's mounted closer to the ceiling, then having a darker ceiling will probably yield a better picture and vice versa.