I didn't like the 150-500, the IQ just didn't seem quite right. I also have had 3 or 4 100-400Ls, and only one rivaled the Sigma 50-500 OS that I own for sharpness at the long end. The Canon has better IQ at the wider end of the lens, but at the long end, 400mm f5.6 vs 500mm f6.3, the Sigma won out.
Through the past couple of years I wanted to lower the weight of my gear and tried to move away from the Sigma, so I would pick up a 100-400L. Each time I did this and compared, between the OS, longer reach and better IQ at the long end, I decided to sold off the Canon. I had people lined up to buy the Sigma too, each time I did this exercise. I gave up and over the past 6 months, finally decided I would stay with the weight/size of the Sigma.
The 150-500 I owned for a while just didn't give me the same results.