I suppose a Warren Burger or a Sandra O'Connor are better than a Ruth Ginsburg or a Stephen Breyer, but not by much.
McCain never said that Alito was too conservative. He said that he wouldn't appoint someone who labeled themselves as conservative because they'd be too hard to get through a Democratic Senate. He did say he would appoint someone like Chief Justice Roberts, who I think has shown to be a fairly strict constructionist.
Now of course there are no guarantees, But the odds are great that Hillary or Obama would appoint far left judges than McCain not appoint strict constructionists. We also have a little history to see what happens when a Republican president attempts to appoint someone seen as a moderate AKA Harriet Myers. That nomination went down in smoke because everyone in the GOP except the Specter wing of the party made a lot of noise and objected. The result was the Alito nomination.