This is a tough one.
I think the CTS-V wagon looks really good for a wagon and it is rare. If I NEEDED the extra space the wagon affords then I think it would be something to strongly consider. On the other hand if I didn't need the extra space I would have a hard time giving up a clean E90 M3 because it is such a great all around car. I would rather have the lighter, more agile, chassis over the extra power of the V just because it is hard to exploit that kind of power on public roads.
If the V is more to your liking then I say go for it. You are the one paying for it and driving it so you should get what you want.