Kirk will have far more than six years to wreak havok on the GOP, if he gets in the Washington establishment will hold him up as a model Republican and protect him. He'll almost certainly be renominated in 2016 because the state and national party will support him no matter how badly he votes, he'll have zillions of dollars as a powerful sitting Senator, and the aforementioned liberal media in the tank for him. Most likely he'd become another Arlen Specter and stick around for decades to score wins for the Democrats and continually discredit and undermine "his" own party, while constantly trying to pull the GOP to the left in caucus meetings.
Alexi is expected to vote with Obama by default and has been so damaged by scandals that most Democrats won't even touch him with a 10 foot pole. Kirk is beloved by the national media and in a position to cause far more damage in that seat. Open your eyes.
Excellent points.