I plan on buying this new 50mm Art to see if it could do what my 50L does. The 50L is meant for 50mm freaks and I fit into that category. It's not the sharpest 50mm but it's built to last through the rigors of pro use. The canon 1.8 is too flimsy, the canon 1.4 isn't much sharper than the 1.8, the old sigma 50mm had horrendous focus issues for me, and the 50L is expensive. In the end, I got my 50L second hand and never looked back because it does the job consistently for me.
As for the Art 50mm, it will be here in no time. Then you can really see what would be better for you.
I'm with you and plan to pre-order later tonight or whenever it's available. I'll use it side-by-side with the 50L and decide where to go from there, but I seriously doubt that I'll sell the 50L, even if the Sigma is amazing. If it's over $1500, though, I may not even fool with it. The 50L is amazing at what it does and out of all the lenses I've sold, this is the only one that caused me any regret, which is why I bought another
EOS 1D X, 5DIII, M + EF 24 f/1.4II, 50 f/1.2, 85 f/1.2II, 300 f/2.8 IS II || 16-35 f/4 IS, 24-70 f/2.8II, 70-200 f/2.8II || TS-E 17 f/4, 24 f/3.5II || M 22 f/2, 18-55 f/3.5-5.6 IS || 1.4x III, 2x III