John McCain thinks Sam Alito is too conservative and wouldn't appoint justices like him (from the Wall Street Journal via NRO):

More recently, Mr. McCain has told conservatives he would be happy to appoint the likes of Chief Justice John Roberts to the Supreme Court. But he indicated he might draw the line on a Samuel Alito, because "he wore his conservatism on his sleeve."

Mitt Romney, on the other hand, has called for more justices in the mold of Alito and Roberts. I don't trust John McCain to appoint conservative justices to the bench.