Having owned a CTS-V wagon I can say that it is the best car I have ever owned. It did everything IMO that you want in a car. Comfortable, fast, handles amazing, sounds great (mine had headers and corsa exhaust, ask track day attendee's about the sound) the ride was the perfect mix of comfort and sportiness, GM hit a home run with the Magnetic Ride Control system. The interior is top notch and don't bother with anything but the Recaro seats, the standard ones don't belong in a performance car. My car was 6M but I drove a few auto V's, my personal taste was the 6M was just more enjoyable to drive. If you want to drag race though go with the auto as it is easier to drive.
The bad is of course what Kirk stated, the gas mileage and small tank. I never saw a range higher than 300 miles even all highway. I never drove mine in bad weather but even on cold roads it was very traction limited so unless you plan on adding snow tires to the mix the V is not an all year DD, I would even be suspect on all seasons being up to the task.
In the end it is up to you but if you want the best all around performance car get the V, if you want the best all around performance car that can also haul a lot of stuff, get the V wagon.