I am sure I could have taken those with a sigma, if it focuses accurately. I am not as hung up in the difference between 1.4 and 1.2 (though 1.2 is nicer of course). I just have not had quite the consistency that I like with some 3rd party lenses. I have both the tamron 24-70 vc, and the canon 24-70II. I love the VC on the tamron, and optically they are very close, however, the AF is so much more reliable on the canon. my sigma 50 1.4, though as reliable as my canon 50 1.4 (sold) still misses focus too frequently.
If you have the funds, its a no brainer. The canon holds its value better, is faster, has better color, and will focus more accurately. This is not a knock in anyway on the sigma, which many argue, based on their needs is a better "value", and for some, the tracking speed may make it even a better lens (though there I would just go for the 1.8, if that is a big issue).
I miss the 1.8 sometimes, when I would just like something smaller to put in my bag, or for traveling, and it is a fantastic lens also.
Decisions, decision. Its good to have options and money I guess.