Normally I'm a big fan of the GTR and it is an awesome machine. But the R8 has the style and performance to back up it up as well. On the track it would be close the R8 handles better I'm fairly sure and the power delivery is so smooth that it is more controllable at the limit.
But the style and interior elegance of the R8 just catapult it ahead.
If it's all about the power then GTR for sure, otherwise R8 is tops.
If you need 4 doors, RS7