The engines are fairly different, though, in smoothness and sound. The Aston's V12 starts with the brief, even whirring of a starter motor, then bursts smoothly into life with a sound so sophisticated and seamless that there is hardly a clue to the fact it's made up of separate impulses. Rev the engine and it gets a little louder but no rougher.
The Jaguar is quiet and smooth, of course, just as its limo character requires. But what surprises you is its guttural growl at full noise, along with the muted but unmistakable whine from the blower. This is a British luxury car with the merest whiff of Nascar available on request.
There are other quirks about the XJ's character. The car's ride is quite a lot firmer and sportier than equivalent Audi, Mercedes or even BMW models. Its suspension rates are variable via a push-push Sport button on the centre console, and there's a Dynamic mode that also varies the steering weight, accelerator response and transmission regime.
The XJ has grip and directional ability to match the very best, along with a sweetly responsive steering system that feels tauter when you select Sport and then Dynamic mode. It's a big car, though, and you sit higher than in the Aston, so it feels a little more cumbersome in tight going.
But the Jaguar's big secret is effortlessness. Attack a long, bumpy bend at speed and the car will scythe through so easily, so perfectly on line, that you find yourself wondering, "Was it the car or me?"
The Aston's ultra-close supercar connections mean that you're surprised, at first, to find it so smooth-riding and sophisticated on the more relaxed of its two suspension settings. The level ride draws benefit from the Rapide's extended wheelbase. You feel it in the agility and steering response, even against the Jag. Meanwhile, the car's competence at coping with potholes is surprising - until you select Sport, when you're in an Aston coupe again.
The Jaguar is better riding, much quieter over rough surfaces and - surprisingly - a little quicker. The Rapide is much quicker to turn, marginally better balanced in corners and more stable under maximum-effort braking.
The Aston feels tailored around you. The Jag's driving position is more conventionally spacious, and has a ultra-modern interior that beats the Aston.
Which to choose? If your heart is in supercars and you don't mind the price, the Rapide is the obvious choice. The Jaguar is simply more conventional - it's special, but less special than the Aston.
To read the full comparison test buy this week's Autocar magazine, on sale now.
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