The real issue surely has to be -- you have a Canon, want a 50mm prime so which one do you get. 1.8, 1.4 USM, Sigma or the 1.2L.
I have owned the first 3 (never the L - could not justify the cost) and I have to say there is a very clear overall winner .... its the 1.8. I rate on Build Quality, Image Quality and Value
Everyone complains about the build quality of the 1.8 and rightly so. It is very ... err 'plastic'. BUT I do not think the build quality of the 1.4 usm is anything to boast about (the barrel wore loose on mine very quickly and the USM is not that durable). The Sigma wins on build quality but it seems you have to be very lucky to get one that focuses consistently (mine didn't). So overall in build quality, as it matters for taking photos, I have to put the Canons ahead of the Sigma but cannot really rank them.
The Sigma was undoubtedly the sharpness king ... when it was in focus (not very often), but the 1.4. and 1.8 were not far behind for me. As between them I find it hard to call. I think the 1.8 has it ... just but that is splitting hairs. Call it a draw. Other aspects of IQ (Fringing, Bokeh etc) I could not reliably tell them apart.
Value for money ... It has to be the 1.8. For me the extra cost of the 1.4 buys a heavier, better looking lens that is fundamentally not very well built and does not deliver any better IQ. The Sigma was just too unreliable to consider long term.