If you are talking motor alone (aside from electronics, etc) then neither the TT or the Diesel have been tested long-term---nobody can answer that until a few more years. Which is why I went with a CPO 4.8--I keep my cars 8-10 years and don't want to take the risk.

__________________
2008 X5 4.8
Jet Black/Black Nevada/Dark Burl Walnut
Adaptive Drive
Premium Pack
Tech Pack
Climate Pack
Sport Pack
20" Wheel Option
Premium Sound
Sirius Satellite
Rear DVD Entertainment
Multi-Contour Seats
Comfort Access
3rd Row Seat
Heated Front Seats
Running Boards
3M VentureShield Paint Protection Film