I have the 100-400 and the 70-200 f4 L. The 70-200 is definitely a sharper lens. But, my workhorse is the 100-400, which I use mostly for bird photography. For me, it produces excellent images and I don't know what I'd do without it. I purchased the 100-400 as a replacement for a Sigma 150-500 F5.6-6.3. That lens is now gathering dust, the differences between the Canon and the Sigma are like night and day.