Canon 50 1.4 usm or 50 1.8/2 or sigma 50 1.4 ex

I haven't used the Sigma so I can't compare that one, though I have heard excellent things about it. However, I have used both the Canon 50mm 1.8 and the 1.4. The 1.8 is an excellent lens for the price, everyone should have one. But the 1.4 is a nice upgrade. Better build quality, better optics, and the extra stop can be a huge help.

You've just got to consider whether the extra cost is worth it!