I realize the question wasn't directed at me, but if I were in Georgia, the answer would be easy: Isakson. I wouldn't support him in the primary, but if he's the nominee, there's no real choice.
The balance of the Senate is at stake, and a Marshall win (even if he's "right" on some issues) might affect the balance, possibly the chairmanship, of every Senate committee. Marshall's very presence, even if he weren't appointed to the Senate Judicial Committee, might make it harder to get Bush nominees to the Senate floor. Isakson, though too pro-choice for me, at least voted against partial-birth abortion, and I'm confident he'd support President Bush's judicial nominees, should Bush be re-elected.
This is probably a grossly inappropriate cliché under the circumstances, but -- don't throw the baby out with the bathwater! Work hard for Isakson's defeat in the Primary -- but if he prevails, he's gotta be your guy, even if you're a single-issue Right-to-Lifer. The election of any Democrat to the Senate, even if he is a conservative on all issues (Marshall isn't) will have the effect of moving us a step (maybe a big step) in the pro-choice direction.