Look through samples and such, and just keep the photographer's skill in mind. Those who own the 50L are likely more experienced than those with the 50/1.4, but looking at the images will show you how the lens "draws" a scene.
I much prefer the Sigma 50 over the Canon 50/1.4, but I still much prefer the 50L over the Sigma. Not because it's sharper or has better color or bokeh or something, but because I like the way it draws. If anything, the 50L is optically worse than the Sigma in just about every category except flare resistance.
But I can't afford a 50L, so I have a Sigma.