The Range Rover Sport has impressed us ever since it arrived on the scene back in 2005, even though it isn't what it claims to be. To a casual observer it seems to be simply a smaller (though not much smaller) version of the full-sized Range Rover, whereas in fact it's much more closely related to the Land Rover Discovery.Why? Well, Land Rover's view was that Range Rover customers might be interested in a relatively sporty model, while Discovery customers probably wouldn't. So, purely for marketing reasons, the Sport was made to look more like the car it isn't than the car it is.Odd though this may seem, the resulting product has been good from the start, and it became better in 2009 when Land Rover thoroughly revamped both models. The curious thing, though, is that while the Sport definitely benefitted from the various changes made that year, it also - to me at least - fell slightly behind the Discovery in the list of cars I might want to own.There is only one reason for this that I can put my finger on. The on-road behaviour of the Discovery made a quite extraordinary leap forward; its stability through corners reached a level that I would not previously have believed possible for something so heavy and tall, and its ability to soak up even very large bumps in the road became not far short of miraculous.The Sport became better in both respects too, but the improvement was less marked. Three years ago, faced with the opportunity to choose between these very similar vehicles for everyday transport, I would have gone for the Sport without hesitation. Today, I would be more likely to pick the Discovery.Not that I would object if someone bought me a Sport for my birthday, however unlikely this may be. Like the "proper" Range Rover it is unbelievably quiet, even though it has to hammer a colossal amount of air out of the way every time it moves. It's also magnificently comfortable, and its recently revised three-litre turbo diesel engine, now producing a maximum of 242bhp, is as fine a unit as could be imagined for the purpose.You don't actually have to have that engine, since there's a 503bhp five-litre supercharged petrol alternative as tested here. This sounds fabulous when you're revving it hard, but it adds more than £10,000 to the price, crucifies the fuel economy and provides far more straightline performance than you could possibly need. Avoid. Stick to the diesel.It's also worth repeating that, although most people will buy a Sport primarily because it's a luxury car, it also performs quite brilliantly in difficult off-road situations, helped by the fact that it has the full Terrain Response system fitted to the Discovery rather than the less comprehensive version found on the Range Rover. I can understand why you might not particularly want to risk scratching the paintwork of a car for which you have just paid over £50,000, but it's an experience to savour nonetheless. Engine 2993cc, 6 cylinders Power 242bhp Transmission 8-speed automatic Fuel/CO2 30.7mpg / 243g/km Acceleration 0-62mph: 8.8 seconds Top speed 124mph Price £55,185 Details correct at publication date