.... I have owned and shot thousands of images on both these lenses on E-M1X and also with MC-14 or MC-20 mounted. Both offer good results with MC-14 mounted but I can't get enthusiastic about using the MC-20 on the Olympus 100-400mm. The MC-20 works extremely well on the 40-150mm F/2.8 PRO though!
The image quality is superior on the 300mm F/4 PRO to the Olympus 100-400mm, also the build quality is noticeably superior.
BUT, a zoom can have big advantages depending what subjects you target. The 300mm AF is faster although the 100-400mm is not a slouch.
High-end zoom lenses are just as good as primes nowadays as technology has advanced but I would describe the Olympus 100-400mm as a very good value for money zoom which isn't high end, as its price reflects.
I sold them both to help buy the 150-400mm TC PRO which is stellar!
The above is just my opinion and Jeff
@the black fox gets excellent results from the 100-400mm.
Here are examples of some of my images on the Olympus 100-400mm :
https://www.flickr.com/photos/114775606@N07/albums/72157715959375717
I hope this helps.