I know there is a Giant dealer that offers an extended test ride of some of his bikes. Too bad you can't try each of these bikes out over a weekend or even a short afternoon near your house.
I have the same reservations you have about tires, but I do know that I tried almost every set of tires on my Trek 6500 before I decided it just wasn't going to be a road bike.
You have two great bike choices there.
Fred