I think if Obama wins the Republicans will return to their conservative roots in 2012, but America will suffer considerably with a very liberal President and a liberal Senate and House to pass all of his measures.
I think if McCain wins, there will be more spending discipline in government, but not enough, and it might prevent needed reform in the Republican party.
The question for me is simply: Would reform of the Republican party be worth the amount of damage an Obama presidency (with liberal House + Senate) could do--i.e. new massive entitlements, harming American businesses, harming trade. Believe it or not, I originally considered going 3rd party when McCain won the primary, because he is watered-down conservatism, though I don't believe he is nearly as bad as GWB spending-wise. After reading Obama's proposals on healthcare, taxes, turning more of our soverignty over to the UN, and ~$1,000,000,000,000 in new spending, I honestly don't think the damage is worth it and I hope he will be defeated.