What's the difference between the spindle assembly checkpoint (SAC) and mitotic checkpoint complex (MCC)?
It can be helpful look into a few technical articles on the subject to clarify these ideas.

For example, this article has an abstract that lays it out quite nicely in the first 2 sentences of the abstract:


  In mitosis, the spindle assembly checkpoint (SAC) ensures genome stability by delaying chromosome segregation until all sister chromatids have achieved bipolar attachment to the mitotic spindle. The SAC is imposed by the mitotic checkpoint complex (MCC), whose assembly is catalysed by unattached chromosomes...


So the SAC is a human conceptual model of how cells prevent the improper onset of anaphase before chromosomes are set up properly for cell division. MCC is the actual protein complex acting as an agent to make the SAC happen in cells. 

The SAC could be thought of as containing the MCC and any other agents that prevent improper anaphase until spindle assembly and chromosome bi-orientation is complete, but it is not a protein complex per se in the sense of a bunch of proteins that stick together for a specific molecular function.