Hi Colin,
There doesn't seem to be much in it (Zeiss V Samyang) in terms of Resolution/MTF and Coma performance.
The Zeiss 135 f/2...
https://www.lenstip.com/388.4-Lens_r...esolution.html
The Samyang 135 f/2
https://www.lenstip.com/442.4-Lens_r...esolution.html
The Zeiss appears to show marginally better than the Samyang in terms of Resolution measured MTF50, but the Zeiss was "measured" with a Nikon D3x (24.5Mpixel) vs the Samyang 135 tested with the Canon 5D MkIII (22Mpixel) so I wouldn't read too much into any marginal 2-3 l/mm resolution difference.
The coma test on both lenses also appears excellent, maybe slightly better for the Samyang, but ....????
Best
JA