I'd get the 50mm if you want to take existing light shots without a flash.

Are you getting the 18-55mm kit lens for general purpose use, too?

If not, you may want to give some consideration to it for a walk around lens when you don't need to worry about taking photos without a flash indoors and/or need something with a wider angle of view compared to the 50mm (which will have a 35mm equivalent focal length of around 75mm when used on a D50).