In theory, the attenuation of a floor or ceiling should be the same no matter which direction you're going through it. However, if you position the access point downstairs, you'll probably be going through the bottom of quite a bit of upstairs furniture to get to your laptop including the desk it's sitting upon. Putting the access point in a clear area upstairs, and then shooting down through the floor goes directly to the laptop, without going through the furniture. Therefore, I would sugget you put it upstairs.
Another criteria is putting the access point near where you plan to use it the most.
Still another is to take advantage of stair wells and put it upstairs near the stairwell so it doesn't have to go through the floor and furniture.
Of course, there's always the infamous, try it both ways and see which one works best.
Incidentally, if you have aluminium foil backed insulation or poured concrete as a floor/ceiling, you're gonna have a problem as RF doesn't go through those too well.