On a full-frame camera such as the D600/800 the 50mm is my workhorse from head shots to full length. The traditional "hallway" shot might be a thing of the past as more cons are cracking-down on photogs blocking walkways. I always ask the cosplayer to step aside where there is a little more working room and better backgrounds. However, if hallway shots are your thing I would suggest a zoom to minimize your frame-up times. The 24-70mm f/2.8 is an amazing lens but it's heavy, expensive, and if you use a flash the f/2.8 speed is not really needed. The 28-300mm is super versatile as far as zooms go.
I chose to use primes because they're light, keep what I want sharp and what I don't blurred, and when night comes are invaluable. They are a PITA to swap-out but hey, that's why I use an interchangeable lens camera.