Originally Posted by bob williams
Wise thinking. [Y]
If you're certain that the 200mm will be long enough, and you'll really use the extra stop, then it's a good idea. However, personally, I find that I don't use f/2.8 all that often for nature, landscapes, and macro. The 70-200 f/4 L IS suits me better for those types of shots because I'm stopped down anyway, and the I.S. gets me from 1/500 to 1/30.
Wildlife is one category where f/2.8 would definitely be useful (except when the DOF is just too ridiculously thin). But it's also the category where 200mm is not nearly long enough. I find that 400mm is barely adequate. But if you're confident that you can use 200mm without cropping then go for it.
I want to emphasize how important it is to *not crop* when using the 70-200 f/2.8. As soon as you start doing that, it would have been better to stick with the 100-400, even at f/5.6. For overall image quality and especially noise. For example, if you shoot 200mm f/2.8 ISO 100, then crop it to the same angle of view as 400mm f/5.6 ISO 400 -- the 200mm f/2.8 ISO 100 will have a lot more read noise ("shadow noise").
Hope that helps.
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