On this episode of Head 2 Head, Motor Trend's Jonny Lieberman pits the 2013 BMW M5 against the 2013 Porsche Panamera GTS in an epic super sedan showdown. The battle begins in California's Mojave desert with a classic quarter-mile drag race. Then Jonny takes both V-8 powered sports saloons up to Big Bear, California to find out which one carves up the winding mountain roads the best.