Because Hillary would unite the republicans against her. McCain would split the party down the middle and get his trash pushed through. That's why McCain would be worse.
No, I don't think he would split the party. First of all, he has NO chance. He agrees with virtually nothing on the platform. His only glimmer of a chance would be going as a VP for somebody very conservative.
Secondly, I bet he's going to become a Dem. He obviously thinks he's too cool for words, and the Dems love him. Wait until he gets into the pack and they eat him alive! Hah!