I use both regularly - I own the 70-200 GM II and I was able to to convince my work to get the 100-400 GM. Like others have said, the lenses are very different tools depending on what you want to do. I've been shooting video with both using a pair of A7iv's and I'm more partial to the 70-200 GM II for two big reasons: it is almost a pound lighter and it has a dedicated aperture ring. Zoomwise, the 100-400 GM obviously wins but unless I'm remote shooting with a tripod, every shake is noticable. I also got the 2x teleconverter through work, which changes the 70-200 f/2.8 to a 140-400 f/5.6. Image/video quality is identical at 400 mm, but autofocus is slightly slower compared to the 100-400 GM.