The interior is just as good: "It's one of the most special cabins of any British saloon of teh past 20 years - Bentleys and Rolls-Royces included."
Comfort sat in the back is the next test. "In the Jag you feel very slightly more confined than you do in the back of the S-class. But there's still generous space to stretch out . The rear bench is fixed, so your seating position isn't as adjustable as it is in the Merc, and although you have seat heaters and coolers, there are no massagers."
The Jaguar XJ's ride also impresses with its "pleasingly controlled high-sped compliance". The Merc beats it on cobblestones and sharp ridges, though.
Behind the wheel, and the Jaguar again defies expectations. "It's unexpectedly responsive and agile," says Saunders. "While Mercedes has engineered in a softness to the S-class's major controls, the XJ's are sports car sharp."
That's not to say the XJ can't be driven smoothly, though - it does 'comfy and laidback' extremely well too.
Out on a challenging bit of road, and with the dynamics set to sport, the XJ excels. "getting back into the S-class and driving down the same piece of road feels like stepping back in time," notes Saunders. "Where the XJ felt poised, eager to please the driver and always willing to go faster, the S-class is unwieldy and unwilling."
Where the Merc does exceed expectations is with all its toys and impeccable material construction, its unerring motorway manners and a more pillowy low-speed ride.
Spec matched, the Merc is almost £2k more expensive.
The conclusion? The Merc is refined, luxurious and effective. But the Jag is a paradigm-shifter. It can be plush and soothing one moment, composed, precise and entertaining the next.
To read the full comparison buy this week's Autocar magazine, on sale now.
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