Thanks everyone for your replies. What I've gathered so far is:
— The Sigma 50/1.4 is probably the sharpest at f/1.4 of the bunch, and nearly a match for the 50L's bokeh
— The 50L has better build quality, weather sealing, bokeh and possibly microcontrast and color
— The Sigma is prone to AF issues
I have the Sigma 35 and it's an excellent lens. It did take two copies to get a good one, though; the first front-focused badly.
I'm trying to decide whether the additional cost of the 50L is worth it. Sounds like I might be better off trying for a good copy of the Sigma 50 first.