100% Warner. If that was the race, I'd leave the Republican Party because the Democrats would officially be more conservative than the Republicans in the ways that matter most to me.
I don't know if I'd become a Democrat because Warner was nominated. Bill Clinton was far more conservative fiscally than Bush has been, but he doesn't make me want to leave the party. Now if the GOP officially abandoned their fiscal roots, then I would.
But I doubt Warner wins the nomination. He isn't left wing enough for Democrat primary voters. Remember, Hillary was considered right wing by a lot of Obama supporters. Plus, he isn't inspirational.