I occasionally use the EX-90 for with a EOS-M camera, and it is barely adequate, especially if you want to bounce. I prefer using the 470ex-ai for when I don't need the 600ex. The 470ex-ai supports zoom range up to 105mm which matches well with the EF/RF 24-105 f/4 IS. The 600ex supports up to 200mm (i.e. 70-200s). The ai is nice because it can reposition the flash head for you when you switch between landscape/portrait orientation automatically.