The differences in image quality between the f1.8 and the f1.4 Canons isn't huge. I couldn't see a big difference in iQ with the Sigma, either.
The Sigma is a little sharper wide open. Both the Canons benefit from a little stopping down, if sharpness is a concern. My Canon 50/1.4 is usable wide open, but I generally try to use it at f2 or f2.2 or smaller.
The Sigma and Canon are about the same from f2 or f2.2 through f5.6. Smaller apertures than that, the Sigma isn't as sharp.
The background blur of the f1.8 is the coarsest. The Canon 50/1.4 improves on that a lot. The Sigma has slightly smoother yet. The Canon 50/1.2L is the smoothest of all, as might be expected. The 50/1.8 has a five-bladed aperture. The Canon 50/1.8 has an 8-bladed. The Sigma is 9-bladed. The 50/1.2L is 8-bladed with curved blades.
The 50/1.8 has somewhat lower contrast and color satutation. This is probably mostly because it's more prone to veiling flare. Both the 50/1.4s are better in this respect, though the Sigma seems to have a slightly cool tint (that's easily changed in post production). The 50/1.2L renders the richest colors of the bunch.
Even though the Canon 50/1.4 isn't "true" USM, it's got considerably better performing AF than the 50/1.8. The f.1.8's micro motor AF is slower, less accurate and noisier. In fact, some who have tried to set Micro Adjust with FoCal software have found it impossible with the f1.8. It's too erratic.
The Canon 50/1.4 has a hybrid form of USM. It performs well, but is a little easily damaged. A hard bump on the focus ring or the front barrel can damage the AF mechanism. Best solution is to use the matched lens hood with it at all times. When shooting, it protects the front barrel. When the lens is stored with the hood reversed, it covers and protects the focus ring.
The Sigma 50/1.4 has HSM, which is similar to Canon USM. So it's a pretty good performer, too. However, it's common for the lens to need to be sent in for calibration when new... or one or more exchanges to get a "good copy".
The 50/1.8 is inexpensive, very lightweight and small. It lacks a focus scale, has a very narrow manual focus ring and is about as plasticky as a lens can be. More than a few have broken in half. Still, for the money it's a bargain... is able to take far better images than the price might lead you to expect. It uses a 52mm filter and, because the front element is somewhat recessed, might be used without a lens hood (the Canon hood is sold separately and expensive because it needs an adapter, too).
The Canon 50/1.4 costs more than 3X as much as the f1.8, weighs about twice as much and is a bit bigger. It uses a 58mm filter and should always be used with a matched hood (sold separately... if the Canon hood is too expensive, get one of the third party hoods).
The Sigma 50/1.4 costs 25% more than the Canon 50/1.4. The Sigma comes with a matched lens hood, so there isn't that add'l cost, the way there is with the Canon. The Sigma is also considerably bigger and heavier (bordering on ridiculous for a 50/1.4, IMO). It uses a 77mm filter and actually is close to the size and weight of the premium 50/1.2L.
The Canon 50/1.4 and 85/1.8 complement each other very nicely. Used on a crop camera, they are a nearly ideal "short portrait" and "long portrait" pair, with similar image look and quality, as well as similar AF performance. I don't see one or the other as "better"... they give different angles of view for different types of portraits. Use the 50mm for head and torso to full length portraits and the 85mm for tighter head shots or to work from a greater distance. If you kept the 50/1.8 and got the 85/1.8, I predict that you'd be looking at upgrading from the f1.8 soon, too, once you see how much nicer the USM lenses are.
Canon 50mm f1.4 at f2, EOS 30D at ISO 400, 1/30. Handheld, ambient light.

Canon 85mm f1.8 at f2, EOS 30D at ISO 1600, 1/400. Handheld, studio lighting (softbox with modeling light only).