sorry for the late reply. Both lenses are great portrait lenses each with its own set of features. It might sound boring but in this focal length, most recent lenses are good enough and stunning portrait depends much more on your model, expression, light, composition and other creative factors, than on the technical limits of the lens.
Sigma is very sharp and has high contrast, while Samyang is softer wide open with more creamy bokeh. Sigma gives you AF ability, while Samyang don’t. Sigma is more expensive and probably better made, but Samyang is made well enough.
My preference will go toward Sigma, because I highly value its almost apo chromatic performance, similar to my Otus 85/1.4.
I hope you are doing well under sea, wherever you are diving now!
P.S. Mozes da pises i Srpski, mnogo postilaca na forum ne dolazi, ne uspevam da ih privucem.