Two Convertibles with more similarities, such as the body structure, market segment, starting price, ground shadow size, height, and maximum occupancy, than differences, like the horsepower and fuel economy. And finally, the 2015 BMW Z4 has a more powerful engine and gets 2.5 mpg less than the 2016 Mercedes-Benz SLK-Class. Among similarities are the body structure, market segment, starting price, ground shadow size, height, and maximum occupancy.