I've owned both. Originally I bought the E-M1 Mark II to replace my Canon gear when it got too heavy for an aging body. The specs and size were right. I found it to be just fine save for a bewildering set of menus whose depth was astounding not to mention frustrating. Coming from Canon I was used to having a "My Menu" that I could customize with frequently used settings. When I heard about the G9 I was intrigued as one of my sons had a G series camera that he used daily for video. I had opted for Panasonic lenses when I got the OM-D so I decided to try it out. The G9 is a much better replacement for the Canons and it has a My Menu! Sold the Mark II and have never looked back. Thinking very hard about the 100-400 lens as well.