Well the Porsche is a better car just about in every way. I doubt the R8 would be faster then even the current 911 Turbo let alone the new one. Sound is debatable but I'd give a slight edge to the R8. But the R8 is one of the prettiest cars ever made IMO so this wouldn't even be close, R8 all day every day.
Now the only problem is the new R8 is coming out in the next 6 months. Although I happen to see it at the Auto Show a month back and it just wasn't as nice
. The back end is very nice but the front end looks all squished up and out of proportion.