The CTS-V is the way to go. Preferably the 05 with the LS6, 5.7 vs 6.0 with same hp. The motor suits the car better. The 04 had it, but that car had some ugly antennas that they did away with for 05. You know the rear end is an issue, but there is an updated version that you can get via warranty otherwise i know a lot of people are using a ford 9-inch rear kit priced around $5,000. The new gm rear still has a little whine to it, but it's much stronger than the original.
There's a comeback with aftermarket parts for the car too, i know Racing Brake is developing a brake kit for it, if they haven't already.
I can't remember the name of the package, but there was a suspension kit from gm that was available on the car which was a direct copy of what the test car in germany had. If you plan on putting the car on a track that kit is the way to go. It's a little stiffer but the cornering is much better and i believe there is a little issue of wheel hop on a launch with the production setup. That package is around $1,500 from the dealer. I can get you the part number if you end up getting the car.
I came really close to buying an 05, but circumstances changed and i needed a truck so i went with the tbss. So i did a ton of research on it and I know a few people who have V's, so even though i didn't get one, i'm still involved with them. So i'm sure i can answer any questions you have.
So hands down, my choice is the V