McCain's office has confirmed that he has an incurable, aggressive form of brain cancer.
Cochran's resignation has been talked about in an unsourced Politico article, and its not even clear if the article in question was in refernece to him leaving the Senate or just stepping down as head of approps. Its possible that he and his staff are still comfortable keeping him on until 2020, nothing has been said.
Obviously, its not a good idea to speculate on these things, but a McCain resignation is much liklier than a Cochran one at this point.
If McCain is forced to resign for health reasons, I think the Dems odds of taking back the Senate go up significantly.