50/1.4 will be quite a bit sharper than 50/1.8 at f/1.8 to f/3.5. The difference is less obvious at smaller aperture. However, the difference in sharpness might not be that obvious if you are not pixel peeping. For me, both are similarly sharp when photos are viewed at the size of my com screen.
50/1.4 is using USM for af. 50/1.8 is not.
The bokeh from the 50/1.8 is actually quite harsh, especially if your bokeh contain grass or flower. Bokeh from 50/1.4 is smoother. Shape of bokeh also different. All this is due to the number of blades being different between the two, or so I read.