Statement of Purpose-How specific it should be?
Disclaimer: I am still a graduate student and my experience is limited to my own application process and as a student member of my department's graduate admissions committee.

I have seen that specific reasons are almost always better than general reasons. When I was going through the application process, my SOP always mentioned current specific projects that my POI's were doing and how I could be an optimal choice for those projects in the near future with my background and current research. This approach seems to have worked well enough for me. 

During multiple interviews in the application process, my SOP was often alluded to, especially in context with these specific projects and my fit with them.