I believe the new 600 plus the series III converters is most versatile but I shoot mainly birds. The weight is almost the same as the current 500 which I know I can hand hold. I also have version I of the 70-200mm f/2.8 and 300mm f/2.8L which I will keep. It just seems to me with birds you need all the focal length you can get, so for me the 600 seems right. (assuming the sharpness and AF will come close to matching the 800)


For action sports the 400 2.8 is the standard ultimate lens, I would think you could use a monopod or tripod if weight is an issue. Carrying around and hand holding the 400 2.8 without a monopod would be a serious workout.