Regardless, these two occupy a similar space in the marketplace and are rivals. The R8 got a facelift for the 2014 model year which endowed it with an optional seven speed dual clutch transmission so no transmission disadvantage for it versus the optionally available Porsche PDK.
The R8 is rated at more horsepower (although less of it likely gets to the wheels due to drivetrain losses) with 430 horses at the crank versus 400 for the Carrera S. So which is faster? The R8 hits 60 in 4.0 seconds flat and goes through the 1/4 mile at firstname.lastname@example.org. Not bad, but not great in this day and age. The rear wheel drive 991 Carrera S Cabrio takes slightly longer to get to 60 due to traction at 4.2 seconds but matches the R8 1/4 mile sprint at 12.5 seconds. Its trap speed tells the story though at 114.1 miles per hour. The Porsche 911 Carrera S is pulling harder past the 1/4 mile and that is impressive for a smaller six cylinder motor. Especially considering it is manual and not dual clutch like the R8 which would have made the gap larger.
For those saying the rear engine layout is antiquated, well, here is an area where it pays significant dividends. The 911 Carrera S stop from 60 in 93 feet exceeding the R8's 101 feet. Very, very impressive.
What about handling? 24.0 seconds at .85g's around the figure eight for the R8. The 911 bests it again though with a 23.7 second time at .88g's.
The 911 is said to be more fun to drive. It wins all the performance categories. A 500 pounds weight advantage definitely helps and that is quite a difference in heft. For someone who wants to drive, it's a no brainer. For someone who wants to be seen (a valid reason for having a $100k+ cabrio), well, that is something the R8 definitely excels at with its less conservative styling. Both are simply great vehicles but the Porsche 911 does more with less.