I own both a gtc vxr, and a vxr8. The difference in the cars is like light and dark, they are both amazing cars to drive.
The R8 is a lazy bruiser that still puts a smile on my face after 5 years of ownership, and in my opinion is a much better drive. Be prepared its a big car and won't feel as safe as the gtc round corners if you are giving it some stick.
I can't really give you anymore help as they are so different, but if I had to sell one of my cars the gtc vxr would be the one to go.