Sharpness. Both the Sony and Sigma lenses have enough apparent sharpness that crosses the threshold of being good lenses to being great lenses
However, at the extreme edges of the frame with the Sigma, there is more of a drop-off of sharpness with photos taken wide open compared to Sony.
The Sony lens keeps things together remarkably well. When stopped down to f/8 in the example below, both lenses are identical in sharpness.
Related to lens sharpness, I’ll add that when I compared vignetting, flaring, and color fringing, I found that all of these are equally well controlled.
Jumping to the section below, you’ll find some ghosting and aberrations crop up in the Sigma f/1.4 and Sony f/8 images.