The crop frame 7D with the 70-200mm lens gives you a full frame field of view of 112-320mm while adding the extender to the zoom on a full frame body gives you a 140-400mm field of view. The big difference, of course, is that the 70-200mm lens on it's own is f/2.8 while with the 2X extender it is f/5.6. The former is needed when lighting is less than optimal but the latter will work in daylight.
The other big difference between the two combinations is that the 7D is designed for fast shooting, the 5D III less so. The addition of the extender slows down the auto focus of the 5D III even more, resulting in an almost unacceptable combination for action sports. As the light faded late in the game, AF was almost non functional.
As usual for sports, I used manual exposure control, adjusting the ISO as needed for lighting changes. Here are are some of the better shots with setting information included in the caption. By the way, the match took place in Schenectady, New York between Hartwick College and Union College
|230mm, f/5.6, 1/100 sec., ISO 200, cropped to 4:3|
|390mm, f/5.6, 1/100 sec., ISO 200, cropped to 4:3|
|400mm, f/5.6, 1/100 sec., ISO 200|
|140mm, f/5.6, 1/100 sec., ISO 400|