I have both the 70-300 is usm and the 100-400L. Of course the 100-400 does have more reach and for birding. That can sometimes be very important. I use both lenses quite a bit. At a recent Blue Angels air show I had the opportunity to use both lenses. I think the IQ of my copy of the 70-300 is excellent. I would rate it just under my 70-200L about par with the 100-400. I especially like it's light weight. Focus is quick enough and sharp. But at 300mm and f-5.6 it is a light hog and it can't use an extender. I also like the image quality of my 100-400L. It gives me the extra reach I sometimes need. At 400mm and f-5.6 it is also a light hog. Since it is heavy It can be a little hard to hold still for those long shots. Also It won't auto focus with a Canon extender. That said I would not trade either of these lenses. Since I don't do just one type of photography both seem to get a pretty thorough work out.