Regardless of how "moderate" the guy is, he's still going to be a vote for Mitch McConnell for majority leader, a vote against the President's budgets, a vote against the President's major policy proposals, etc. McCain is supposedly a "moderate", but he's still pretty much been voting against everything the Democrats have proposed. Castle, remember, voted against Obama's budget and the stimulus, just like every other House Republican.
McCain is obviously not a moderate. I think most people have finally realized that and he's never been a moderate. Castle, on the other hand, has a history of being a moderate. However, as you said, he's still with the GOP on key votes.
