I prefer the diesel because of the low RPM torque. Driving through the mountains on vacation, it climbs most steep upgrades with nary a downshift... it just pulls and pulls as if it was a gentle slope.

Now, I also have a Z4, and I would not want a diesel in my Z4. But for the X5, the diesel's Torque/RPM curve is superior to a gasoline engine, in my opinion.

A 500 miles plus range is also quite nice when traveling on vacation, e.g. I averaged 26-29 mpg on the highway, cruising most of the time at 65-75 mph.