I went through plenty of 50mm. I would have to say that the Sigma Art is your best bet. Yes, at f2.8 this lens is as sharp as my 70-200ii at f2.8. At f1.4, it is plenty sharp and is sharper than my 85f1.2 and especially at the corners. The sig50 also has far less CA too. For sharpness, I think you will be very happy with it. However, I have the USB dock and have calibrated my Sigma to 1,1,0,0. If you get the sigma50, get the dock if you are looking for absolute autofocus reliability.
Before, my go to primes were the 35mm / 85mm pairing. Now, I only bring my sigma 50mm. The autofocus is fast, consistent and trustworthy. My 85mm has again became a specialty lens. My 35 is my indoor specialty lens. The 50mm is really versatile and the SigArt50 is the first 50mm that I am truly happy with.
As far as auto focus speed, I would say the sig50 is as fast as the 35L but not as fast as the 70-200ii. But it is plenty fast. I never had an issue with the AF speed.