Here is my experience within the 70-300mm range. My first purchase was a used 70-300 IS (non-L) lens. It took excellent small perched bird pictures, usually racked to to 300mm. Its down side was the dramatic extension as it zoomed out (potentially sucking in dust), and it rotated as it focused, neither of which I liked. I then got the 70-200mm f/2.8 IS II, which is a fantastic lens. It doesn't extend, it doesn't rotate, it takes great pictures wide open, and the AF is very fast. Later I traded my 70-300mm plus $500 for a 70-200mm f/4 IS. I have to stop the f/4 lens down to f/5.6 to get the kind of sharpness of the f/2.8 wide open. The IS in the f/2.8 is much quieter, and the AF is noticeably faster than the f/4 lens. If I am walking fewer than 5 miles, I don't mind the weight of the f/2.8. Shooting indoors with existing light, the f/2.8 wins every time. I have both the 1.4X III and the 2.0X III teleconverters, and the 1.4X works well on both L lenses, is small and doesn't weigh much. Birds in flight (BIF) focus speed degrades noticeably with the 1.4X, while the 2X is basically useless if you lose the frame and the focus get confused on the sky. If shooting static shots, both extenders work well, of course the 2X only works on the f/2.8 lens. For BIF my choice is the 400mm f/5.6 and my 6D. This is the only combination that has worked for me, taking pictures of hawks and eagles as they take off from a perch. (Other bodies I own and have tried for BIF are a 5D Classic, 40D and 60D.) If shooting more general (larger) wildlife, the 60D, 70-200mm f/2.8 & 1.4X TC are fine, and have about the same reach as the 6D and 400mm f/5.6. If outdoors in good light the f/4 zoom is a good walk around lens, particularly on a full-frame body. I have never used the 70-300 L, but I don't plan to buy any more variable f/stop lenses. Often my 15-85mm f/4.0-5.6 is racked out to 85mm and is too slow (or too much ISO noise) indoors. Optically the 15-85mm is just fine outdoors with good light. It generally stays on my 40D. All these comments assume we are not discussing cost, just the pros and cons of each lens.