Up. the light will reflect on the ceiling and bounce from above to flatter all. If down, which i see too much of, the light just points down to the sink. dont highlight the sink. You want to highlight your reflection and skin in the mirror, that I'm assuming is coming. Up is my vote

For best light, there should be side sconces, that will light the sides of the face. But usually there's no room on the sides of the mirror, so we have to have lights above. The only way to tell what you like is to install your mirror, and look at your reflection with the lights up, and then down. If you're applying make up here, you're going to want the most light.

side sconces are a good idea.

Down- I hate mine that are up and collet ever bug! I live in the country, so more bugs, but still....... eww..

http://www.homedepot.com/p/Hampton-Bay-Dakota-4-Light-Satin-Nickel-Bath-Bar-BR-4DAK-HBU/203197525

check this out.