I'll have to admit the biggest reason I didn't buy a diesel X5 is my 335d has had really subpar reliability (or is it bmw typical??). Its had a CBU Cleaning at 21K miles, EGR valve replaced 3x, EGR cooler replaced 4x, MAF 4x, Exhaust pressure Sensor 4x, DPF replaced 1x, SCR metering valve 1x, SCR tank 1x, NOx sensors 2x, vacuum hoses, red boost hose (replaced with forge), etc. The car has only 52K miles on it and has a service record rap sheet like a career criminal ;-) ..... Fortunately all was covered under warranty, no extended warranty on the car, not worth it at the prices I've seen, tune and delete.

So after that I was like, gasoline for my tow vehicle. I still like my D especially with its tune, but damn the thing spends almost 50% of its time in the service bay at the dealer. I'm a big participant on the M57 forms at e90post and my reliability issues are not unusual, though mine may be worse than average. Case in point I have a fellow D owner with 90k on her 2011 335d and she has just spent 6K recently fixing the car; injectors, vibration damper, egr cooler, CBU cleaning.

The 4.8i isn't the most relaxed towing engine, this is where I think the D with its superior torque at around 2000rpm or a 5.0i would be a better choice.. The 4.8i has to be kept revving all of the time at 3200-4500 to stay in the fat part of the torque curve so as keep my speeds constant on hills and on flat stretches. The 4.8i makes noticable noise at full throttle in 5th to make the uphill grades and can't keep a constant 65mph in 6th even on flat interstate. The wind resistance is killer, with my trailer being 10 feet 11" tall and 8 feet wide sail, thats significantly larger than the frontal area on open trailers or boats. Thats what is killing fuel economy for me. I suspect the X5 D would still be about 9-10mpg with my trailer which is about 20-25% better fuel economy then I get in my 4.8i. Boats are better on fuel economy, because of their low profile and hull shape.