Last year I used a 300mm prime f2.8 and got some great shots. This year I've been using a 200-500mm f5.6 for all my long range sniping and have been happy with the results.
However, the clouds came rolling in and I had to increase the ISO to get the higher shutter I needed to freeze the action and get the f stop to keep the auto focus on song. Increasing the ISO can have a detrimental affect on the quality of the image, some camera bodies handle it better than others.
I must use the 300mm prime more but is so heavy, the 200-500mm is light and has the reach but AF performance suffers in low light.