A couple of very important questions
Do either of you plan to boost your horsepower and torque with mods like flash programs, large FMIC, catless downpipes,etc?
If so both the auto and the manual transmissions have proven to take a lot more hp and torque than their rating. If you have a manual you will need to replace the clutch if you are hard on it like blasting off from a dead stop but will take a good while. Auto is just as robust and it too will wear out clutch plates.
Bear in mind that "typical" clutch life for a BMW is around 110-125K miles based on normal use. Hiway driving will extend that and city driving will hit that mark so as long as you know that will be coming up.
On the auto your life expectancy is related to how often you change the Lifeguard 6 transmission fluid. If I just purchased a 50-60K mile 335 auto first order of business is to change all the fluids. Do more research and see what will help you keep this ride healthy...