Despite its hefty, 5,040-pound curb weight, the ML350 Bluetec moves out well, with power in reserve at midrange speeds. The diesel's power characteristics are well-suited to towing, and the SUV is rated to tow up to 7,200 pounds when properly equipped.

The diesel's stout low-end torque enables this; it produces 455 pounds-feet from just 1,600 rpm, which is considerably more than the gas V-6's 273 pounds-feet of torque that arrives at a higher 3,500 rpm. All versions of the SUV have a seven-speed automatic transmission and all-wheel drive.

Diesel engine refinement has come a long way, and the M-Class' diesel V-6 is evidence of this. It starts up quickly, even when outside temperatures are hovering around freezing, and idles smoothly. You can hear a little bit of diesel clatter when outside the SUV, but once you step inside it's sufficiently muted. The engine sounds a little bit different from a gas engine when accelerating, but it's close enough that your passengers won't suspect anything — until you pull up to a truck stop for fuel.

The diesel's fuel economy advantage over the gas V-6 is a nice benefit: The EPA rates the diesel at 22 mpg in combined city/highway driving versus 19 mpg with the base gas engine, a 16 percent improvement. I got around 23 mpg, mostly on suburban streets and highways with little stop-and-go traffic. Depending on how much diesel fuel costs where you live, some of the money gained from better mileage could be lost at the pump. A gallon of diesel cost $4.39 when I filled up in suburban Chicago, and nationally it's 17 cents more than premium gas, which the gas M-Class uses. There's also the matter of the up-front expense for the diesel M-Class, which starts at $1,500 more than the base gas version.