1. The Viper costs about $25,000 less
2. The Viper has 180 more horsepower
3. The Viper has 214 more ft/lbs of torque
4. The Viper has an 8.4 liter motor compared to the Audis 4.2 liter (there is no replacement for displacement)
5. The Viper gets better fuel economy. 13 in the city/ 22 on the highway compared to the R8 that gets 12 city and 19 on the highway.
6. The Viper weighs about 150 lbs less. Viper weighs 3454lbs compared to the R8 3605lbs.
The R8 does have all wheel drive and carbon fiber though.