I don't think there is any argument, it's a considerably better lens. It's much sharper and bokeh is much better. It's well built. Focuses pretty fast.
The Sigma 50 is 'arguably' better than the L but at the expense of 0.3 stops of Aperture.
The Sigma 85 I think is sharper than the L, it focuses MUCH faster and is much lighter, has nice bokeh, but again misses out on the 0.3 stops aperture advantage.
The aperture advantage is mainly going to affect DOF rather than light gathering so much (Silicon not good at absorbing light from angles).