Give 50L a chance.
You did not just say the 50L is anywhere near the Sigma. The 50L, though performing perfectly from f/2 upwards, is one of the worst lenses price/performance-wise, it's just really, really soft from below 2.0, even the old 50 1.4 performs better in that range. And this is not just based on test charts, I owned it myself for quite some weeks before I switched to the razor sharp Sigma 35 1.4. There is quite a quality scatter but not even close as worse as it was with their old 50 1.4 and if you get a decent copy - like i did - it's just a treat to work with. Give it some time, get to your local lens dealer, and pick a lens by hand. You won't regret.