EF 100-400 or Sigma 150-600 mm

[Seahorse]

I have an inbound tax refund I would use rather than SWMBO, a 5D MKIII and a Sigma 50-500mm 1:4.6-6.3 APO DG HSM which is is OK, but the sharpness at the long end isn't that good, essentially it's the oldest and 'worst' lens I own as well as being the longest. Main current uses are occasional motorsports, wildlife, yachting, moon, and other things that are too far away for me to chase/swim/fly to!

I am looking to replace it and having read tons of reviews and the relevant threads here I can't decide between a Canon EF 100-400 f/4.5-5.6L IS II and Sigma 150-600 mm f/5-6.3 DG OS HSM SPORTS. Size and weight aren't an issue, but other than the obvious 200mm and 1.6 stops, how do the IQs compare? Any other obvious benefits or issues with either option that might help me decide?

 

[Vertigo1]

The Canon is better, so it basically boils down to whether you need the 400-600 range. If you do then get the Sigma as the Canon won't do it at all, otherwise get the Canon.

Personally I'd get the Canon (and plan to next year) and get a 1.4x to go with it for when you need even more reach.

 

[the black fox]

What your shooting is all reach affected ,so personally I would go for the sigma I have had one since they came out and the performance is stunning ,they also work well with a canon 1.4 tc giving you even more reach albeit on on f8 cameras ,not sure about the 5d3 but I think it's o.k .there are tons of photos availed from the sigma to compare there's a dedicated thread on here to to check it out .
While the canon may be marginally sharper it lacks that 200 mm of reach ,also the thing I love on my sigma is the ability to tune it to your own needs via the USB dock

 

[Jackwow]

I don't know anything about the Sigma 150-600 but I did have a 150-500 and I really wasn't that impressed. Now I have the Canon 100-400 Mk2, which is superb and works well with a 1.4x Mk3 tc, albeit at f8.