I think the new 150-600 siggy has better IQ than the 50-500. I did notice that the 150-600 had bit if vignetting at 600mm. 100-400 Mk2 is nice lens but I didn't test with 1.4 TC
The S or the C of the 150-600? . I shoot on7dMii/7D so dont normally notice vignetting.
I'm looking at also renting a 150-600 C for a winter yellowstone trip in Jan for a second body. My girlfriend and I finally decided to stump up the dollars for a semi private (4people) week long photography tour of yellowstone in winter .