Quick thoughts:

The R8 is the better overall car. It seems more connected, more refined, and honestly, it looks exotic and its still fast. The GTR is brutally fast. I've never driven a car that fast. Its shocking and in a straight line. Handling seems slightly disconnected. Its kind of like riding a rocket.

I could live with either car. However, the R8 does it for me - I'm biased, and I admit it. I've always been an Audi guy. But the R8 is by far a more livable car and still carries that exotic car feeling thats not easy to come by. Will the GTR kill me in a drag race... yes. But its rare that you'd come up to one anyways. So I enjoy the overall feeling of the car much more. Plus there's something awesome about hearing the engine behind you. Plus that manual gate shifter and the sound of the engine (from outside at least) is far more melodious and inspiring.

The GTR is stupidly fun. It shouldn't be allowed to ever drop into a teenagers hands. This car makes no damn sense when you drive it. The acceleration is stupendous when the turbos kick in. The handling is great, but feels disconnected. Like I'm wearing a pair of gloves a size to big. The interior is not even close to the R8. Not close. The navi and functions, though, are better than the Audi. The exhaust sounds overly mechanical, as does the engine. I felt like I was listening to a factory machine and not an incredibly potent engine.

I think you can't go wrong either way. Both cars offer something for their owners. The GTR is ridiculous if all you care about is going fast - nothing else will match it for that price. But I care about the overall car, and the R8 (even the V8 version) does it for me. I'm not the type of person to track the cars, so for me the V8 R8 is the sweet spot at that price range.