@Colin-R
For the best signal spread these should not be mounted on the wall or ceiling, instead positioned centrally in the area, however that is usually not an option.
In terms of Wall vs Ceiling... ceiling is a tad better mainly due to the ceiling being wood / plasterboard and not as "heavy" as the walls would be. Signal tends to pass floors better than walls from experience.
However from years doing this, dont worry about it and just choose what suits you best for cabling or aesthetics. The difference between walls and ceiling is so small its not worth getting too concerned about.