2 completely different cars the GT-R should be compared to a porsche the Audi an m5, If it's thrills your after and Supercar status on the road then the GT-R
If you want something fast and stealth then rs6, they are very very boring to drive though interior is a much better place but how much comfort does someone need in a car
I can put you in touch with someone what owns a GT-R as a toy and rs6 as a daily if you want