I have been saving for over a year now (and still have about a year to go) to invest in a Super Telephoto for the purpose of professional wildlife photography. I have read Bryan's reviews and done some research. Iwas wondering if anyone else has used these lenses to know which is a better choice. It seems like the 500 f/4L is about the sharpest lens Canon makes, and while the 600 f/4L has the extra 100mm (very useful for wildlife), it wieghs much more (bad for backpacking), and costs $1,400 to $1,600 more (bad for my wallet). I am also looking to do some full field sports photography, primarily football. Is the 600 f/4L worth the extra money and weight? Any advice, or additional expertise would be great. Thanks.