Why are comparing two different cars? What are you look for in a car? If you are look to just have fun and drive around with no more than two people then get a Z4. But if you need a car to get groceries and drive you family around in then get an X5. The X5 is very good at holding the road for an SUV and the Z4 isn't the best sports coupe, if I were I would look at an Audi TT with the 3.2 instead of the Z4.