BMW 6 Series 640D M Sport

Price: £66,745
Engine capacity: 3-litre diesel
Power output (hp @ rpm): 313 @ 4,400
Top speed (mph): 155
0-62 mph (seconds): 5.5
Fuel economy (mpg): 51.4
CO2 emissions (g/km): 144

Four seats, buckets of power, sleek lines, comparable pricing and a front-engineered, rear-wheel-drive layout... You could be forgiven for thinking there's not much to choose between the new BMW 6 Series and Jaguar's ever-popular XK coupé. But imagine for a moment you're a banker. You've managed quietly to pocket a hefty bonus without the tabloids striking camp on your driveway and you're looking for something sporty to park on it. An Aston Martin or a Ferrari is out of the question – your bonus isn't what it once was – so it's either a Beamer or a Jag.

The diesel 640d in the M Sport trim is set to be the most popular variant of the new BMW 6 Series and is the obvious choice. At nearly 5m long, the BMW is a big car. And its long, sloping bonnet, 19in alloys, black brake callipers and rear bumper give just the dose of aggression our brash bankers will be looking for.

As usual for BMW the 640d is packed with technology – far more than the Jaguar – but it is its potent diesel engine that really impresses. One minute the twin-turbo is a snarling charger that revs like a petrol (and is one of the best sporty diesels I've driven), the next it is a comfortable cruiser returning more-than-respectable fuel economy. And that economy hints at greater practicality. The ride is smooth, the rear seats almost practical, the cabin refined and the emissions sensible. All very German, but perhaps a little too dull for our boy-racer banker's sporty wheels...

Jaguar XK Portfolio

Price: £70,550
Engine capacity: 5-litre V8
Power output (hp @ rpm): 385 @ 6,500
Top speed (mph): 155
0-62 mph (seconds): 5.2
Fuel economy (mpg): 25.2
CO2 emissions (g/km): 264

...In comparison, there's nothing dull about the Jaguar XK. It has little of its German rival's practicality but the motoring blogs have been thick with comparison and it's still the benchmark to test any coupé against.

Some still think of Jaguar as the car of choice for the driving-gloves generation, but the firm has worked hard to shake off that reputation. Coming soon is the sleek C-X16 hybrid supercar, and the XKR-S, Jag's hottest XK yet, with a 542bhp engine, has just gone on sale.

The standard XK is where I'd be tempted to go with my £70k. Unlike the 640d it's a brash petrol V8 with more power, but higher running costs and poor fuel economy. It has a delicious growl and then a menacing roar when you drop the gears, floor the throttle and explore what it has to offer. With smart lines and plenty of leather, shiny metal and soft fabrics it feels every inch the classic British (but, erm, Indian-owned) sports car. And while the 640d can feel large and unwieldy, the XK feels sharp and agile with just the right amount of driving feel.

Yes, our banker's calculating brain should probably opt for the sensible BMW, but since when did bankers start making sensible decisions?