Two Convertibles with some similarities, but otherwise different in most aspects such as the horsepower, model year, fuel economy, height, and maximum occupancy. First of all, the 2014 Z4 has a more powerful engine than the 2016 TT Convertible, and it gets 3.4 fewer miles on a gallon of fuel. Also, the TT Convertible is slightly taller than the Z4. In addition, you'll seat two more passengers in the Audi TT Convertible than in the BMW Z4. Among similarities are the body structure, market segment, starting price, and ground shadow size.