Until very recently I owned all three lenses, but I sold the 400 L. The image quality of the 100-400 II is better in the center and by far good enough in the corners. Contrast is better. IS is sooooo much better. 400 L is lighter and has less distortion, that's about it. The 70-300 is nice if you want to travel lighter. Image quality of my copy is perfect even on the 5DsR. So I take the 100-400 L (+ 1.4x converter) if I need he the reach, and the 70-300 L if I don't need the reach and want to travel light. After have not regretted selling my 400 L, although it is without a doubt a superb lens.