Here is the comparison of Panasonic Lumix S Pro 50mm f/1.4 L-mount lens with Sigma 50mm f/1.4 DG HSM Art lens. Richard Wong has compared build quality, autofocus performance, image quality (Sharpness, bokeh, flare, distortion, CA, vignetting..etc) of these two lenses.
From the video above, it seems that Sigma 50mm f/1.4 DG HSM Art lens has better image quality and sharpness. The US price for Panasonic Lumix S Pro 50mm f/1.4 lens is $2,299 (Buy at B&H Photo Video/Adorama/Amazon), the US price for Sigma 50mm f/1.4 DG HSM Art lens $949 (Buy at B&H Photo Video/Adorama)