I don't have either the Canon or the Sigma, but the Nikon 1.4 is far smaller and lighter than I expected, and to be honest, in my opinion it's too small. It feels cumbersome to hold the camera.
In that respect alone I'd have paid the extra for the Sigma, and it is apparently the slightly better all-rounder too. When buying I didn't think it would be worth the £100 or so extra, but in hindsight I probably would have splashed the extra. Don't get me wrong, the Nikon is an excellent lens and other than the size of it, I've no complaints... just that niggling thought at the back of my mind that maybe I ought to have swung the other way.
Ironically I do have several other Sigma lenses (the excellent 150mm 2.8, the 30mm 1.4 which is my favourite lens, and the 17-70mm... which is very average overall, but a good kit lens replacement for a crop body), yet I passed on the opportunity at maybe their best lens.