As people have said it's really an apples and oranges kind of comparison.
The GTR is a lot more brutal as a DD and touring type car. It's loud(I disconnected the BT because I felt like I was 90 saying "huh?" all the time) and harsh compared to the 6 series. If you can deal with that go for the GTR. The performance is in a different league.
If you can't, I'd say wait for the M6.
BTW, I'll give you a deal on a 2012 GTR once the 2013 arrives