Up until recently the question of getting an original branded lens (e.g. Canon, Nikon) vs a 3rd Party lens (e.g. Sigma, Tamron) was not really a question. If you could afford the original brand lens, the answer was, go for the original.
However, with Sigma’s recent ART lenses things may have changed. Photographer Will Chao took both the Sigma 50mm f/1.4 ART lens and the Canon EF 50mm f/1.2L USM Lens for a head to head battle and the results are not as conclusive anymore.
The photo at the top of the post and the photo below are an easy daylight test for the lenses.
Another thing I have noticed is that although F1.2 is only 1/3 to 1/2 of a stop faster than F1.4, the exposure difference is in fact around 3/4 of a stop, this is likely due to lens coating that may have affected transmission (? need confirmation), but let’s see how it fares against the sun.
And the photos to match it:
Will has some more samples over at his post, which I recommend checking out (some are mildly NSFW).