While the 100-400 is about 4 years older than the 70-200/2.8L IS, all
bets are off when it comes to guessing which would see any design
changes...so my recommendation is to buy what you know is going to
maximize your utility, not which one you believe is going to stay
"newer." What's a missed opportunity worth?


Regarding the preference for telezooms over telephoto primes, I can appreciate your point, but the problem of course is that you're not left with a lot of options. The 100-400/4.5-5.6L IS is pretty much it, and it is the only 400mm zoom in the Canon lineup. If anything, I'm surprised you don't already own it given your stated shooting preferences--it's impressive even in its current incarnation.


My personal belief is that the importance of focal length is overemphasized in popular photography relative to aperture. By this I do not mean that I think one can get away with using a 24mm lens at an outdoor sporting event, or a 800mm lens for panoramic vistas. Having a variety of focal lengths at one's disposal is essential. What I mean is that the trend of technological improvement in zooms has led to an over-reliance on adjusting focal length to establish composition, to the detriment of sharpness and perspective considerations. Now, I'm not one of those 'prime shooters' (I love my zooms), but I've noticed in my own shooting style that I often subconsciously end up using the extremes of the zoom range and actually walk about to frame properly. And of course that makes absolutely no sense because I could have done better if I had chosen a prime to begin with! So I recognize that issue in my own experience.


Aperture-wise, I tend to shoot very wide, even with ample light, because I like to use shallow DOF to establish areas of interest. There simply isn't any other way to make that kind of image. As a result, I'm quite comfortable with the idea of sacrificing some zoom flexibility if it means I can shoot wider apertures. Of course, YMMV. It all depends on what you like to shoot, and how you go about the process.


I wonder what a 200-400/2.8-5.6L IS would look like. Certainly it could be sharper (though not necessarily smaller) than the 100-400.