I'll add my twopence.
When you are spending that kind of money and you are not sure, you can only try them out for yourself to ensure they meet your requirements. I can offer some past experience when looking to buy a 300 prime where I went Sigma initially, but soon went back to Canon.
Before anyone out shoots me down! I must stress again that this is a personal experience and does not represent Sigma lenses as a whole.
3 years ago I wanted a 300mm prime and had 2 lenses in mind. Sigma 300 EX f/2.8 and the Canon 300 f/4 IS
L. I didn't get the chance to try the Canon, but at a show I was able to test the Sigma and it appeared to be a good lens. I bought one!
Optically I could not fault it, the results were excellent. However there were downfalls that soon became apparent. The autofocus was awful. It would hunt, it was slow and if shooting through a fence, as there was no AF
distance settings it kept hunting and the shot was lost. I thought I'd just have to live with that, but then the lens casing near to the drop in filter became lose and internal lens seals starting dropping on to the optics. I contacted LCE Southampton where I bought it and they requested I took it back immediately. They offered me a replacement and when I mentioned I was not happy with the AF
and would like to try a Canon they suggested I took this one for a week and if not happy come back for a refund or swap. I took it to Duxford and got some great results, but the AF
was still very poor, I had also lost confidence in the product. I went back to the LCE tried a Canon 300 f/4. The difference was amazing, the AF
was pin point and fast so I changed.
I remained happy with that lens and recently upgraded to the Canon 300 f/2.8 IS
L and that is simply superb.
I can't believe that all the large Sigmas are like the one I had, I had a bad one but that experience sticks and I wouldn't spend that kind of money on another. My Canon 300 f/2.8 cost me double the price of the Sigma, but it was well worth it in my mind. I am also confident that if I ever sell the Canon, I know I'll get a good price. I've sold other L glass and there is always a good demand for well looked after L lenses. I wouldn't share the same confidence selling a Sigma.
Ultimately it's your choice and this should really be made on the basis of which lens meets your requirements and through testing them extensively, not just for 10 minutes in a store. You wouldn't spend €6-7k on a car without driving it first.