There are, as I write this, ten different versions of the M available from the, also as I write this, four Infiniti Centres dotted around the UKThe diesel is quiet, refined and, with a maximum of 235bhp, powerful. That last figure puts it 81bhp shy of the V6, but thanks partly to the diesel's far superior performance at low revs it's only 0.7 seconds slower from rest to 62mph. Both cars are limited to a top speed of 155mph so there's no advantage either way if you find yourself looking for some sport on a deserted autobahn.Furthermore, the diesel's 37.7mpg on the combined fuel economy cycle makes the 27.7mpg of the V6 petrol look rather foolish, and substantially lower CO2 emissions make it £325 cheaper to tax in the first year and £190 cheaper annually from year two onwards. Since there's nothing about the petrol engine, fine unit though it undoubtedly is, which begins to make up for all this, I'd have to say that for me it would be the diesel every time.Then there's the question - if you're not going to waste a second of your time considering the un-named entry-level model - of whether to go for the S or the GT. According to Infiniti, the S is the sporty version and the GT the luxury one, and it's certainly true that the S has far more of the whiff of the race track about it. Among other things, it has sports suspension, 20" wheels and four-wheel steering, none of which is available on the GT.Ah, but hold on a minute there. That suspension and those wheels (or, rather, the very low-profile tyres fitted to them) mean that the ride will be absolutely terrible, which can't be said for the GT with its 18" wheels and more sensible rubber. I drove the GT on a wide variety of roads, and it covered the ground exceptionally smoothly even where the recent sub-zero weather had created havoc with the tarmac. It was all rather restful.At the same time, though, the GT could be made to hustle along twisty country roads in a more sporty fashion than I could ever have guessed in advance. Yes, it feels like a luxury saloon, but it turns into corners well and maintains a nice balance all the way through them. Yes, of course, an S, with its mechanical advantages, would have been quicker, but I didn't want to go quicker. And an S would have been much less comfortable, and by that time I was rather getting used to comfort and considering it a fine thing.So far, the fact that the car reviewed here was a diesel in GT trim was all working out rather well. But as you'll have gathered from the heading of this page, there's something I've missed out, and that's the fact that this car is actually a GT Premium. Premium (which is also available on the S) is a result of Infiniti noticing that when customers were asked which options they would like, the answer was usually, "All of them."To save them the bother, Infiniti created the Premium package, which adds a little over £5000 to the list price and includes the Dynamic Safety Shield (a package of features including Blind Spot Intervention, which helps prevent the M being steered into another vehicle alongside), Forest Air (a glorified air freshener, more or less), a 16-speaker Bose sound system and the Connectiviti+ information and entertainment system with 30GB hard-drive satellite navigation.A couple of comments about that lot. First, the satnav display is very swish, and announces that you have reached your destination by playing a little fanfare which shook me to my boots the first time I heard it. Unfortunately, it sounds like something Casio would have rejected as being too cheesy in about 1992, and I'm sure a better alternative could be arranged.Second, the Blind Spot Intervention system is very clever and greatly to be admired, but I can't help thinking that if Infiniti really took this sort of thing seriously it would have made the blind spots less of a problem in the first place by doing a better job with the design of the windows. Infiniti considers styling to be of major importance, and seems to work on the basis - on all its products, not just this one - that visibility is a much lower priority.Still, from one angle the view is rather good. Look straight ahead and you'll see that the bonnet is framed by those large, gently curved front wings. That's a very pleasant sight, not as dramatic as the one a Porsche 911 driver enjoys but quite evocative all the same. The interior is even more abundantly blessed with wide-radius swoops, giving a soothing effect which the surprisingly plain instrument design can't entirely spoil.Despite being an enthusiast of fast driving, I don't believe I would enjoy the sportier S Premium nearly as much. The diesel GT Premium's comfort, ease of operation and general feeling of well-being work for me. Engine 2993 cc, 6 cylinders Power 238 bhp @3750 rpm Torque 406 ib/ft @1750 rpm Transmission 7 speed semi-auto Fuel/CO2 37.7 mpg / 199 g/km Acceleration 0-62mph: 6.9sec Top speed 155 mph Price From £44372.00 approx Release date 01/09/2010