The car in front (Audi R8) has all-wheel drive, which means the power is actually sent to all four wheels. Ceteris paribus, AWD gives more grip, as all four tires are putting down power, therefore giving the car a more stable disposition in hard corners. AWD cars typically respond with understeer when pushed hard, which means it won't turn as much as you want. This is safer and more predictable.

The car in the rear (Porsche 911) is rear-wheel drive, which means power only goes to the rear wheels. Ceteris paribus, RWD gives less grip, as only two tires are putting down power. RWD cars typically respond with oversteer when pushed hard, which means it will turn more than you want, snapping the rear around. This is a little more dangerous, but for those that like it, much more fun. This car has it's engine mounted in the rear, which makes it even more tail-happy than the average RWD car.

Now, if you look at the rest of the thread, you'll see there's much debate on which is better/faster. Each has pros and cons, but everyone has their own opinion. They're both pretty awesome in my eyes, but with four of them in the driveway, I'm a disciple of RWD.