Hey Eric, I enjoy wildlife photography as well. I currently use the 70-200 2.8 IS and it is a great lens, but for wildlife I find that I am always wanting something alittle more. I have used the the 70-200 with a non-canon 2x and found that it did focus alot slower, and the images were noticable degraded at 100%, but not unuseable (and that was with a cheap 2x). I personally have not used the 100-400 but if you were opting more towards wildlife I would choose the 100-400, from what I have seen in reviews its a great lens in its own right. The push pull zoom does take a moment to get used too though, but in the end I thinkyou will like the results