I just moved to FF, and my canon 50mm 1.4 isn't so grand anymore, it's still sharp where it needs to be, but the bokeh really could be a lot better. On crop the lens performs nicely wide open, the center sharpness and what bokeh you get is smooth, but with the whole image circle I don't like the harsh background, also need 8+ afma. My Sigma 70-200 OS is beautiful in comparison, and convinces again that the Sigma 50 would be nice, sharpness is important, but I would rather have creamy bokeh than the best sharpness when I'm already shooting wide open. Since, if I'm going to be stopping down so often then why not get a zoom lens, one gets a fast prime to shoot fast right?
Longer reach lenses Generally melt the background away more than 50mm lenses (other factors being similar) ... thus show their Bokeh more clearly. At less than 80mm, you need really large apertures to get same quantity of OOF Blur. I had the same feeling with the EF 24-70 f2.8 ii vs the EF 70-200 f2.8 mk.ii. The Tele had so much better OOF blur rendition..... which ofcourse was amplified by the increased Quantity of Blur due to the longer reach of the Tele's.
If you want sharpness and don't care much about Bokeh, you can get the 50mm f1.8 with it's world class penta Bokeh
. If you are into Portraits (where sometimes softness and Bokeh make happier customers than pore magnifying ultra sharp lenses) then the EF 50L 1.2 is hard to beat. It was Engineered to do just that. The Sigma is a nice balance between sharpness and Bokeh. Depends what you want...
If your Budget is $750, Perhaps think about a used 135L, if thats too long, the 100 F2 or 85 F1.8 do a nice job for a lot less money. The 100 F2 is very Sharp and has a nice Bokeh, the 85 f1.8 does the same for less $$ and both outperform the EF 50mm F1.4 IMHO.
The longer reach lenses will give you better working distance (discrete away from subjects making them less conscious) , they have more blur, and best, they have better compression
than the wider angle 50mm which can sometimes suffer from some barrel distortion which is more noticable at shorter focal lengths. Heck I sometimes had to correct the 85L for barrel distortion that made my subjects looked like they had larger noses than they really had.... this is wher ethe 135L can do good (or 70-200mk.ii)
In the end it depends what you want it for....