I can only speak of the lenses I have owned: 50mm f/1.8 II, which you already have, and the EF 50mm f/1.4. The f/1.4 is certainly better. It might be sharper, but the 50mm f/1.8 II is awfully close (and very good in that respect; I can't flaw it for that) but the photos have better bokeh and the colour seems better. The contrast might be a bit better too. I am happy I upgraded. It's not perfectly sharp wide open, but it's acceptable. Mine is much better than many people indicate here in the forums. I can successfully shoot portraits wide open with it.
I should say, though, like the Sigma, buy yours from a cooperative retailer. The first 50mm f/1.4 I bought wouldn't focus worth beans on my 5D. I replaced it with another, and all was good.
The Sigma 50mm might be better in some ways, but I think it depends upon what you are doing with it. From the reviews I've seen, the Canon 50mm f/1.4 is significantly sharper in the corners stopped down. I use my 50mm for landscapes as well as portraits, so I need that. This is just what I've read, so take it with a grain of salt.