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 model year, horsepower, and fuel economy. First of all, the 2013 Z4 has bigger and thirstier engine; the 2016 SLK-Class gets 3.5 more mpg. Among similarities are the body structure, market segment, starting price, ground shadow size, height, and maximum occupancy.