Coleman.
Perhaps, something tells Brown that in a good year for the GOP, he'll be out on his arse.
Brown is a ferocious campaigner, and he'll be tough to dislodge as an incumbent. If the Republicans get their own "2006" in six years, he'd be in trouble; but he'd hold on. After all, what will they attack him on? Everyone in Ohio knew that he supported queer marriage, mandatory sodomy, live-birth abortions, Islamic terrorism, and so on... they voted for him anyway.
His economic positions are very popular in Ohio, and that will keep him in office.