I am doing a project on face detection in C#. I want to find the skin area by using skin color segmentation. For that purpose, I have to extract the skin area.
I'm not an expert, but judging from the Wikipedia articles, YCbCr is used for image compression (JPEG and television [YPbPr]), while HSI is used in computer vision. Since you're doing computer vision, if we believe Wikipedia then you should use HSI. Of course, you could always experiment with both representations and all six dimensions. See what works best. If you're fitting a linear model, then it doesn't really matter, and you can even use RGB.
Ycbcr just has one component (y) for luminance and two for chrominance and it is less light variation sensitive than HSI, and for face detection Ycbcr comes to better result