Choice is a great enabler but on occasion, you have to wonder whether we're being offered choice for the sake of it. Take Mercedes for instance. With a huge and occasionally dizzying Range of models, there's bound to be a model to suit you. Probably two or three actually. The latest M-Class luxury 4x4 is burgeoning in the number of trims and engine choices available but Mercedes has taken a rather odd tack. They're offering the petrol ML350 and diesel ML320 CDI models within a whisker of each other pricewise and they've just reprised the same trick with the petrol ML500 and the diesel ML420 CDI.

Having run the rule over the ML420 CDI, I'm having genuine trouble seeing how Mercedes can sell one single ML500. Seriously. The ML420 CDI is beefier, way more economical, will realise a better resale value and removes that final weapon from the armoury of the committed petrolhead - it's even better to drive. The clincher is that Mercedes has priced the two cars at an identical £50,435.

Let's pause for a moment in the adulation. That's a lot of money, especially when the ML320 CDI, a vehicle that offers much of this car's capability, starts at just £37,220. You've really got to want that extra power and prestige if you're going to be willing to hand over that big premium for the 420. On any objective basis, the smaller engined car is the better buy but market research conducted by Mercedes has shown that M-Class owners usually have a serious amount of disposable income and, as such, it's easy to see why the ML420 CDI will rack up some respectable sales figures.

After all, Mercedes have stolen a march on both BMW and Audi, who will persevere with six-cylinder diesels rather than the V8 which sits beneath the bonnet of this 302bhp M-Class. Even Range Rover has been pipped to the market, the TDV8 engine's 272bhp looking a little asthmatic next to the brawny Benz. With a massive 516lb/ft of torque at its disposal, the Merc's engine is second only to Volkswagen's freakish V10 TDI when it comes to twist action.

A back to back comparison with the ML500 is illuminating. Let's start with economy. The ML500 driver will require a featherlight right boot to get anywhere near its quoted 21.1mpg. Drive 'normally' and you'll be looking at closer to 17mpg. With the ML420 CDI, you should get to within a couple of mpg of its quoted 25mpg figure without having to freewheel down hills. While this still won't make it the apple of the green brigade's collective eye, it's better than you'd expect from something like a Ford Focus ST. Emissions aren't eyewatering, with the big Merc returning 299g/km of carbon dioxide (ML500 319g/km).

"Drop the hammer and the ML420 CDI responds with genuinely punchy acceleration"

It's properly quick too, the diesel engine and all-wheel drive system acting in concert to catapult the ML420 CDI off the line to notch 60mph in 6.5 seconds, the sort of sprinting ability of a decent GTi. A 145mph top speed will make interContinentalcruising easy while the 7G-Tronic seven-speed auto slurs beautifully between ratios. If you want the best automatic gearbox in the world, you're looking at it right here. Advantage Mercedes.

Although the cabin is a whole lot smarter than M-Classes of yore, there is still the odd plasticky bit that doesn't pass muster. On the whole, however, the interior styling is well judged and fit and finish is tighter than ever before. Much of this has come from a change in factories. Whereas the quality control of the initial batch of cars produced from the Tuscaloosa plant in Alabama were rather patchy, later models from the Austrian facility at Graz have proved to be a whole lot better screwed together.

The current generation M opts for an evolutionary look, keeping the same sporty stance and raked forward C-pillar but adding more pronounced wheelarches and a rising swage line along its flanks that may look a little too Toyota RAV4 for more conservative tastes. It marks a definite swing towards the sports end of the sports utility market and differentiates the Mercedes nicely from cars like the Land Rover Discovery and the Volkswagen Touareg.

Both the front and rear suspension has been thoroughly revised for this generation M-Class, with a double wishbone front and a multi-link rear arRangement. For those that really want the ultimate in ride quality, Mercedes also offers an Airmatic air suspension system option that teams up with an Adaptive Damping System that adjusts the responses of the shock absorbers According to the situation in hand. There's also an optional off-road biased air suspension option which means that the M-Class will now ford water up to two feet deep. More importantly, it adds a two-speed transfer case with an off-road ratio, manually or automatically selectable diff locks and a version of Airmatic which raises ground clearance by 110mm to as much as 291mm. However, our experience with this set-up on its highest setting was that the ride was extremely stiff.

This stiffness in the Mercedes chassis is probably the biggest impediment to its capability in the rough. Wheel articulation isn't great and tyre performance on standard road rubber is a limiting factor. However, ultimate ride height isn't bad, the engine's torque response is excellent and the traction control system is very effective when descending. On the downside, there are notable pauses when the ECU struggles to shuttle drive to the wheels that can best deploy it whilst climbing.

Overall, the Mercedes ML420 CDI is an extremely capable car. It's unlikely to be taken off road but even when judged against road-biased rivals, it still makes a convincing case for itself. While it may not be the pick of the M-Class Range, it gives its direct opposition a real headache. Choices are only problematic when they're not clear cut. The ML420 CDI shouldn't take you too long to pass judgement on.