A rapper can be an MC, but an MC isn't necessarily a rapper...
But all this is garbage without qualifying words such as "good" and "bad" i.e. a good rapper isn't necessarily a good mc...
For example, Canibus is a good rapper, but he is not a good mc...Why? Nobody is checking out for a Canibus bar, verse, song, mixtape or album...
So, how can anyboy say Canibus is a "Master of Ceremony" when the majority of fans aren't celebrating his music today...
In comparison, Chief Keef is a good mc, but not so good at rapping...Chief Keef can get the fans going, but he doesn't really have the rapping abilities of a rapper like Canibus...
In conclusion,
Any rapper who can get the attention of the fans is a good mc...So, a good mc isn't necessarily a good rapper, however, a good rapper isn't necessarily a good mc. That's why we have people like Pac, Nas, Biggie, Jay Z, Eminem and rappers of that echelon as our GOAT candidates, because these rappers are/were good mc's (sold plenty of records) and are/were also good rappers (they influenced the culture and are admired and or respected by many fans and their peers).
That's my take on this whole debate...