The BlueTEC diesel boasts more torque than the ML550's hefty V8: 400 pound-feet from 1600-2400 rpm vs.   391 from 2800-4800.   And it's torque, not horsepower that gets you going and pulls trailers up hills.   (With both properly equipped, the BlueTEC tows the same 7,200 pounds as the ML550.)

The new ML450 Hybrid delivers the best mileage in M-Class, its principal advantage in the city where it betters a like-performing ML350 by 6/4 mpg.   The ML350 BlueTEC diesel, which goes as quickly, tows substantially more and weighs less for better handling, delivers the same EPA estimated 24 mpg highway and 18 in the city.   Note that in our experience with other Mercedes diesel cars and SUVs the EPA numbers have been pessimistic.  

Fuel economy for the gasoline-V6 ML350 4MATIC rates 15/20 mpg City/Highway (16/21 2WD).   The ML550 is rated 13/18 mpg.   The ML63 AMG gets 11/15 mpg.  

Mercedes estimates near-identical 0-60 mph times of 8.0, 7.9 and 7.8 seconds, respectively, for the ML350 BlueTEC diesel, ML350 gasoline V6 and ML450 Hybrid.   Yet that doesn't tell the whole story, because where the gas engine might jump across the crosswalk quicker the diesel comes on with serious urge and rarely needs a downshift in traffic or on the highway when an ML350 does.   Potentially noteworthy, turbocharging maintains power better in high-altitude environs though not like it did years ago.  

The ML550 is more fun to drive.   The 5.5-liter V8 engine will take this 2.5-ton SUV from 0-60 mph in 5.6 seconds, and then settles down to a background burble by the time you reach seventh-gear overdrive.   The seven-speed automatic offers a manual-shift mode, and it's the best automatic Mercedes has built.   It's flexible, in that it kicks down to a lower gear more quickly than its predecessors, and it almost always keeps the engine in the most productive part of its power band.   Yet its overdrive top gear makes for quiet high-speed cruising and better fuel economy.  

During several days of hard driving in mountainous, sinuous terrain, we found the ML550 a hoot.   The suspension is good at minimizing body roll in high-speed corners, and it smoothes dips and potholes well.   The standard 19-inch wheels and tires and the power rack-and-pinion steering deliver a nice feel of the road, and quick reactions when necessary.   At high speeds through mountain passes, the ML550 leans over a little, takes a set, and then grabs the ground and turns the corners; the all-wheel drive system helps keep it going where you point it.  

Critics have complained with some justification about the electronically actuated and modulated Sensotronic brake system that Mercedes-Benz has been feeding gradually into all of its models over the last few years.   We're getting used to them, and they'll stop the M-Class right now without much pedal effort.   They've also improved considerably since they were first introduced.   Yet they don't have the smooth, linear feel of the best mechanically actuated brake systems.   Nice smooth stops can be tricky without practice, although we can't say how feel is affected in the Hybrid which does battery regeneration while slowing.  

On the trail or sand, the M-Class offers typical electronic advantages: a Downhill Speed Regulation software control that maintains the 4-12 mph downhill pace you set through the wheel controls, and a Start-Off Assist that keeps the vehicle from drifting backward in Drive or forward in Reverse on steep hills.   Very handy for novices and easy to use; many more experienced four-wheelers will leave it off but they'd be buying a Land Rover or Jeep instead.  

All no-hybrid M-Class models are rated at a significant 7,200-pound towing capacity with the optional Class III towing package, but note the tongue weight rating may be the limiting factor.   We have little doubt that the diesel and 550 would be up for the job, as would the ML63 but its 20-inch tires aren't ideal for trailering, and would reserve the 350 gas and 450 for watercraft or dirt-bike trailers of less than 4000 pounds.  

We've driven the ML63 AMG and it certainly has appeal: a people hauler to do battle with Porsche Cayennes and BMW X-M's in the stoplight derby or blast through big sweeping curves much faster than decorum or good sense might suggest.   The ML63 is fast, dashing from 0 to 60 mph in less than five seconds, and, with the torque of a large V8 and the high-revving character of a sports car engine, it can be fun to drive.   But it's also big and heavy, so don't think sports car.   Through the twisties where we sampled it, changing directions quickly and often, the ML63 AMG is not exactly fun.   Its massive tires mean plenty of grip.   The steering is fairly quick for such a hefty machine, but also quite light, and the package conspires to feel twitchy, almost unsettling.