I own a 2006 ML350 (gas) and typically get 21-22 mpg with mixed city/highway driving. On the highway, I am getting 24-25 mpg. Not a hyper-miler but am gentle on the gas pedal.

I have driven a 2010 ML 350 BlueTec (Diesel) and have not been able to get more than 28 mpg on the highway. The emission systems on the new BlueTec's are so complex that I would not own one. Add in Run-Flat tires and no spare tire and I am really not interested.

Personally, I would only consider the gasser ML as there is no way (even with better gas mileage) that the Diesel would be less costly to operate.

The only positive consideration would be higher low rpm torque if you were using it to tow. However, it is still not worth the extra maintenance and repair expense of the diesel engine.