What's new? Which prefix to go with your hot Six? The official nomenclature of M? V10 engine, 500bhp, four-and-a-half road test stars and – woof – a carbonfibre roof. Or perhaps B? The B6, of which this is the first in the UK, is Alpina’s semi-official take on the 6-series, with 493bhp and a supercharged V8. Semi-official? Yes. BMW first gave Alpina the nod when it developed a dual-Weber carburettor for the 1500 in the early ’60s and the partnership has been developing nicely since. Today, Alpina provides bespoke parts to BMW, which in turn fits them to 6-series shells on the production line. What the B6 doesn’t get is any M-Sport parts – no carbon roof, body stiffening or aluminium components here; the B6’s basis is a standard 6-series.Regardless, on paper the M6 and B6 are about as closely matched as rivals get. They’re a whisker apart on horsepower, both manufacturers claim 4.6sec 0-62mph times and prices are within a gnat’s eye of each other: BMW asks £80,755 for the M6, Alpina will relieve you of £79,950 for a B6.So there is a genuine choice, and which you go for is entirely determined by the differing characters of the two cars. The M6 is hard-edged and focused, while the Alpina is marginally softer, fulfilling more of a grand-touring role.The B6’s 4.4-litre V8 is loosely based on the recently superseded 645i’s motor. It retains BMW’s Vanos variable valve timing, but gains a nautilus-type supercharger: efficient, quiet, yet effective. There’s genuine urge right from idle and phenomenal pace from the mid-range onwards – it feels every bit as quick, if not urgently responsive, as an M6. Peak torque is 516lb ft, arriving at 4250rpm, while the 493bhp comes in at 5500 revs. You’d need an extra 2250rpm before getting the M6’s peak power. The BMW is unlikely to return such good economy as the Alpina, either. The M-car tends to manage mpg figures in the mid-teens, while the Alpina hits the low 20s, giving a useful advantage over the M6’s pitiful range of under 200 miles. The B6’s automatic gearbox is more convenient than the M6’s automated manual, too. It is a six-speed ZF unit and a terrific thing: it changes gears smoothly and shifts intelligently in full-auto mode, while there’s also a manual override, via the gearlever or two nipple-like buttons on the back of the (nicely sized, thin-rimmed) steering wheel.What's it like? Alpina’s suspension tweaks mean that the B6 rides with reasonable compliance most of the time. It is a little nuggety around town and transmits thumps on broken surfaces, but is otherwise acceptable.Where the B6 can’t match the M6, however, is when you put the suspension through a sterner test. Perhaps because spring and damper settings are intended to retain compliance despite the car’s 20in alloy wheels, the B6’s body control can be found wanting when you’re pressing on. It fails to settle quickly enough over some cross country bumps and camber changes.That said, the B6 is still capable of some impressive cross-country speeds, with outright pace more governed by conditions and road laws than the car’s ability. And while there’s little genuine feel through the steering, what the B6 does offer is a consistent, nicely judged steering weight, with excellent accuracy and a linear response.At the limit of grip the B6 has a tendency to understeer mildly, but our B6 came with a new limited-slip differential (standard on the B6 Coupé, optional on the B6 convertible and M5-rivalling B5 saloon). So instead of spinning excess power away through an inside wheel like a B5 does, the B6 is far more throttle adjustable and can be easily coaxed into a predictable and controllable slide. In this respect, the B6 is every inch as enjoyable and exploitable as the slightly heavier M6.Should I buy one? Which is where we came in: M6 or B6? For the good of both companies, their different characters mean there’s ample room for both cars in the market. And while the M6’s keenness and agility make it the superior car, there’s also something particularly satisfying about Alpina’s more relaxed approach to performance driving.