The Sony FE 50mm f/1.2 GM Lens vs. Canon RF 50mm F1.2 L USM Lens comparison shows these two lenses sharing a length spec and the Sony lens shaving about 3mm from the Canon lens's diameter. Basically, the size of the lenses is not differentiating. From a weight perspective, the Sony is 18% lighter. The 6 oz (172g) difference will be modestly noticeable.
The Sony lens has 11 diaphragm blades vs. 10. The odd blade count number will generate 22-point starbursts when used at narrow apertures vs. 10 points. Which is preferable is a matter of taste. All else equal, more blades result in rounder bokeh effects at narrow apertures.
The Canon lens features a focus limiter switch, enabling a shortened focus distance range for potentially less focus hunting in some scenarios. The Canon lens has a slightly higher maximum magnification capability, 0.19x vs. 0.17x.
The Canon is the more expensive lens — $300.00 more expensive as I write this. Will the higher price be justified? The optical performance comparison potentially holds that answer. From what we've seen from Sony GM lenses so far, the optical performance of the new FE 50 f/1.2 should be similar to that of the impressive Canon option.