If I were John McCain, I'd probably pick Kay Bailey Hutchison. I think he's still deluded enough to think he can sway Hillary voters, and he's more than happy to pander in a way that Barack is not. Barack willingly left him that opening by opting for Biden. Imagine how easy it will be play the "picking on a girl" card after the notoriously contentious Veep debate! I believe Hutchison is also an anti-Roe Christian, which helps him with the social cons, and being a Texas senator, she's one of a tiny number of senators that loves Bush even more than he does, which helps him with his troglodyte base. Plus, Hutchison's seat is about as safe as it gets, should she ascend to the White House.
These things may well apply to Sarah Palin as well, though I'm not as familiar with her. Hutchison is almost certainly a more experienced debater, though, and he's going to need that. Then again, Palin would likely be more content to be a curio in the White House, a mere talisman to conjure scorned Hillary voters, while Hutchison might be a little more insistent on taking part in his Administration.
Still, I'd be shocked if McCain doesn't pick a woman.