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This might make it easier to decide:
70-200 f/4 IS
1. smaller, lighter, and for me, much more likely to take with me. Good for travel.
2. good for landscapes because you don't need f/2.8 for landscapes, and the IS works great! No tripod needed.
3. pretty good for outdoor candids because if the smaller size, but you won't get the background blur in most cases
1. Better lens for sports because you can get the shutter speed up and get good subject isolation.
2. Heavier, larger, and for me, less likely to carry. Not a good travel lens due to size.
3. Great lens for portraits (good subject isolation) and some indoor action (with enough light)
4. Very conspicuous, not a good candid lens, draws attention
5. Image isn't quite as sharp as the f/4 IS