Both lenses have their strengths and weaknesses, so there is no clear cut "better" lens between the two. Each person will need to weigh all of the factors and decide for themselves which are more and which are less significant.

Here's a basic rundown of the advantages and disadvantages for each lens:

Canon EF 50mm f/1.4

• Very good overall image quality. Sharper in the corners at wide apertures than the Sigma.
• Less expensive. It currently runs about $350 in the U.S.
• More consistent focus accuracy on Canon bodies.
• Susceptible to damage if the front of the lens is impacted while focused at short distances. For more, please see: Canon 50mm f1.4 no longer focuses reliably. This can be mostly avoided by always insuring the lens is at infinity focus when not being used.

Sigma 50mm f/1.4 EX DG

• Very good overall image quality. Slightly sharper in the center but noticeably softer in the corners at wide apertures than the Canon. Sometimes that look is more desirable, sometimes it is not.
• More expensive. It currently runs $500 in the U.S.
• Third party lenses can be a risk that they won't be fully compatible with future camera bodies.
• Has been widely reported to demonstrate highly inconsistent auto focus accuracy on even upper tier Canon bodies, especially in lower light at at wide apertures. Less of a factor on cropped APS-C bodies than full frame, but still a significant consideration.

For more, please see the excellent reviews for each lens at The-Digital-Picture.
Canon EF 50mm f/1.4
Sigma 50mm f/1.4 EX DG

The Sigma review was written later and includes direct comparisons between theses two lenses as well as the Canon EF 50mm f/1.8 II and the EF 50mm f/1.2 L.

For my own analysis of the major differences between the EF 50mm f/1.4 and EF 50mm f/1.8 II, please see this answer. In regards to the newer EF 50mm f/1.8 STM, a lot of the disadvantages of the older f/1.8 II have been addressed, but I'm not a big fan of the manual focus-by-wire system that all STM lenses employ.