Thank you for your fast answer. I tried to combine GM and WM now and it looks a bit better. Nevertheless the result seems to be a little edgy for me. Furthermore there are some small disjointed parts, as you can see in the image.
Maybe I can handle this by smoothing the surface.
My goal is to compute some shape features like total curvature and maybe some others. When I look at the surface model in *.hemi.mid.cortex.svreg.dfs it looks smooth so I hoped to get such an accurate model as well to increase the accuracy of my features.