i don't think McCain will be able to get out of the primaries given the current climate of the Republican party and the need to play to the evangelical base. McCain is simply too moderate. let's not forget: this man was personally destroyed by Rove & Co in South Carolina in 2000 via a whispering campaign that included the lies that Cindy McCain was a drug addict, McCain himself was unstable, and that he had fathered a black child out of wedlock. while his profile has been elevated in the past 4 years, not least by his shameless campaigning for Bush (who he is said to personally despise) in 2004, he's done nothing other than be moderate and reasonable and hawkish. by refusing to, say, make a diagnosis via videotape on Terri Schiavo in order to pander to the histronics of the Republican base, McCain might be already dead in the water.
as for HRC, still too early to tell. i think, politically, she's got all the right moves, has been able to deliver nuanced but clear positions and effectively triangulate hot button issues like abortion and seem sane when embracing "parent" issues like violent video games. however, she's not a terribly effective public speaker, and while she's gangbusters at pounding home a message and sticking to talking points, she's got none of her husband's charisma and soulfulness.
while i think McCain is the best the Republicans have to offer, personally, i don't think he flies. i think HRC is a great asset to the Democrats, but as a Senator. i'd rather not see her run.