It really depends on what you want the pictures to look like. I recently shot the last two groups of an outdoor festival and used two bodies: A 1.6x crop body with a 70-200mm f/2.8 lens and a full frame body with a 24-105mm f/4 lens. I chose the 24-105mm lens over my 24-70mm f/2.8 lens due to image stabilization on the 24-105mm f/4. I had stage access and the stage was a temporary outdoor one that was less than rock solid in terms of stability. The intensity of the theatrical lighting was highly variable. The spotlights on the lead singers was much brighter than the rest of the stage lighting.
Here are a couple of images taken with the 24-105mm lens (both images are the same resolution, but this page apparently re-sizes based on width, rather than maximum dimension):
24mm, ISO 3200, 1/50 sec @ f/4 from rear side stage
55mm, ISO 2000, 1/320 sec @ f/4 from the media pit
Here are a couple of images taken with the 70-200mm on the crop body:
200mm, ISO 3200, 1/1600 sec @ f/2.8 from front side stage
70mm, ISO 3200, 1/320 sec @ f/2.8 from side stage