35mm 1.8G or 50mm 1.8G on D5100

One of the best bits of advice I read on this issue is to look at the photos you have taken with your 18-55 lens and see what focal lengths you have used for various types of pictures. Think about how you use your camera and lenses and figure out which one would best fit your needs. Each lens has its uses, you might even end up getting both lenses eventually.