Jon asks what gives me the sense that, all things being equal, McCain would rather face Obama than Clinton? I get that sense because, when you hear McCain talk about Obama and Clinton, there's a real difference in tone--not that unsimilar from the difference in tone when he talks about Huckabee and Romney In both instances it doesn't take a mind-reader to figure out which candidate he likes and respects more (unless you think McCain's delusional enough to think that some kind words for Huckabee will win him the allegiances of Huck's evangelical backers).
That said, all things aren't equal, and McCain would obviously prefer to take on Hillary in a general election because she'd be an easier candidate for him to beat. I just think he respects Obama more.
--Jason Zengerle