I was in a similar predicament as you are, a choice between the Sigma or the Canon L. I even considered the Canon f2 as a more reasonable lens.
I decided that the f2 was out of the question, I would always be wishing I had waited and gone for the f1.4. I am a firm believer that if your going to do something, do it right. So it was a 1.4 I was after. Then next thing for me was to try the lenses out. A quick stop at a
LCE store and I could try the Canon and the Sigma.
I tried the Sigma first, weight was good, construction good and the image quality was good. From that point of view couldn't fault it. But when I tried the Canon, it just felt right. I cant explain it.
I then managed to pick up a
second hand Canon L for a very reasonable price when in New York. So I think I got the best of both worlds.
My advice is to try out the lenses ans see what you think. Sometimes you have to go with your "gut" feeling on a lens. I'm so happy I did.