A quick look.
Our local shop has just got in the two Sigmas so I compared them briefly with the 300/2.8 II + 2xTC III, all at 600mm and the 100-400mm II + 1.4xTC III at 560mm on the 7DII. TDP has the 300/2.8 II + 2xTC III at f/8 better than the rest at f/8. However, www.objektivtest.se measuring MTFs accurately has the Canon at f/5.6 better than the Sigma S at f/6.3 and both the same at f/8.
Comparing shots of far targets on a bright sunny day with very fast shutter speeds, I find the two Sigmas to be inferior at f/6.3 but at f/8 all four lenses have little to choose among them optically.
The big differences are in IS and weight. I found at 600mm the image rapidly jumping around on both Sigmas. The 300/2.8 + TC is relatively stable, and the 100-400 II + TC even more stable. I found it very tiring to hold the Sigma S pointing upwards at 30 degrees, even without the heavy lens hood, much more so than the 300 + TC + hood. The C was OK. The 100-400 + TC was a joy to hold in comparison.
So, all 4 lenses deliver approximately the same goods optically. The choice then depends on what you can afford and you can carry.
Our local shop has just got in the two Sigmas so I compared them briefly with the 300/2.8 II + 2xTC III, all at 600mm and the 100-400mm II + 1.4xTC III at 560mm on the 7DII. TDP has the 300/2.8 II + 2xTC III at f/8 better than the rest at f/8. However, www.objektivtest.se measuring MTFs accurately has the Canon at f/5.6 better than the Sigma S at f/6.3 and both the same at f/8.
Comparing shots of far targets on a bright sunny day with very fast shutter speeds, I find the two Sigmas to be inferior at f/6.3 but at f/8 all four lenses have little to choose among them optically.
The big differences are in IS and weight. I found at 600mm the image rapidly jumping around on both Sigmas. The 300/2.8 + TC is relatively stable, and the 100-400 II + TC even more stable. I found it very tiring to hold the Sigma S pointing upwards at 30 degrees, even without the heavy lens hood, much more so than the 300 + TC + hood. The C was OK. The 100-400 + TC was a joy to hold in comparison.
So, all 4 lenses deliver approximately the same goods optically. The choice then depends on what you can afford and you can carry.