As gercajr said, diesels tend to last longer for the reasons he stated. However, the X5 diesel has two turbochargers, which adds more complexity and temperature and speed-stressed parts and bearings, e.g., while a typical normally-aspirated Diesel might last longer/be more reliable than a typical normally-aspirated gasoline engine, I'm not sure a dual-turbocharged Diesel will last longer/be more reliable than a normally-aspirated gasoline engine.

Turbocharging is a neat trick, but it does not help reliability.