The initial plan was for OpenTK to provide low-level OpenCL bindings and to develop Cloo as an object-oriented, higher-level wrapper. At some point, it became obvious that it would be more efficient (reasonable) to disable OpenCL low-level bindings from OpenTK to Cloo.
To answer your question: use Cloo for OpenCL and use OpenTK for rendering. They represent various but complementary projects.