1+ for the Sidewinder. I have a double boom tube, but I think it would be difficult to scratch anything in the cradle.
Two drawbacks: 1) it does not look terribly cool when sitting atop the car with out your tandem.
2) you can make a very big mess if you drive into the garage with your bike on top. (I know you are thinking: I'll never do that...I thought the exact same thing when I was deciding between roof transport and some other option.)