I own a 2009 G8 GXP and have two kids (13 and 9) along with an infant...the backseat room in this car is awesome. I have also been able to directly compare it to a 2009 CTS-V sedan that a family friend owns.
For space, the G8 wins, hands down...if you can't afford the GXP...look at the GT. I have put about 10K miles on my GXP and really enjoy the car's performance. I have a 7 year, 100K full warranty on mine, so I am limiting my modding to a CAI for now.
The CTS-V is a really kick-ass execution and fun to drive. I drove the car stock, and again after the owner modded it...makes around 625 rwhp now...but is very civilized. There are a few downsides, including smaller (but functional) rear seat space and price of wear items (brakes, tires, maintenance, etc.) Granted, it isn't a Corolla...so that is to be expected

The owner of the CTS-V bought it new (about 4 years ago) and has around 42K miles on it with no major failures or complaints. What is concerning him now is being modded with coming close to end of powertrain warranty. As long as they is some warranty on it, shouldn't be a problem to own the CTS-V.