I don't think you necessarily get "better picture quality" from a wind angle lens, but you do get a wider angle. I agree that it's possible to get a huge expanse of foreground or sky but it's also possible that that is what you are aiming for. You can also get some distortion, but you can also correct it or avoid it to an extent. These are not faults, they're effects.

I took my Sigma 12-24 out at the weekend and I got some shots that I'm happy with at zoom lengths from 12 to 24mm. I think that it would be worthwhile looking at a lens in the 10-20 range but I don't think that I myself would be happy to use the phrase "better picture quality." But you may get a shot that may not be possible with a less wide lens.

If you get a Sigma 12-24 you will be able to use it on a 5D in the future too.