Prime lenses work well in low light situations, I have a 50mm but I don't find myself using it that much with cosplay photographs. I mostly use 14-24 f/2.8 and 24-70 f/2.8.
But like sukotsuto said, the 70-200mm would work well if you plan on photographing cosplay contests/skits. I used that lens at a past con and got pretty good results!
Online Cosplay Community: www.elitecosplay.com
2014 convention list we will be photographing coming soon!