Get Sony 135mm f/1.8 GM at B&H Photo.
This page compares the Sony FE 135mm f/1.8 GM to the Sigma FE 135mm f/1.8 DG HSM Art from f/1.8 through f/8 on a demanding planar (flat) target with extremely fine details. Focus shift and field curvature are assessed along with sharpness through the aperture series.
Includes images up to full camera resolution with crops and extensive analysis.
There are important differences between these two lenses; anyone contemplating either should take note.