Just spotted this so I thought that although I'm too busy I might as well have a break and put my twopence worth in.
I have an XK8 that I bought last Christmas (as a treat for myself) It is not to be recommended for any one thinking of cheap motoring and in true British fashion I could list all the bad points for you!
There is however a slight draw back to any faults -. It is a simply a really nice car to drive and have fun with.
Classy in an old money sort of way with a lovable quirk or two that tells you that despite the development over the years they haven't felt it necessary to bother and iron out the faults too quickly! I always manage to open the boot with my knee when I get out of the car for instance.
Mechanically the Mercedes is I would think more reliable - I also have a Merc Sprinter (For moving the motorbike to events) that has never let me down but whilst it s a great van to drive with lots of power it kind of says Munich not Monarco.
The Porsch is a great car but if you are anything like the size I am it is kind of tight!! I settled for buying my wife a V6 4x4 Golf which kind of makes up or the acceleration boy racer in you. It is faster off the mark than the Jag but the top end speed is about the same. So the best of both worlds.
To conclude all can say is that it is going to be a difficult choice when I think about changing. I have not had as much enjoyment with a car since my old BM 635. I thnk that ithe XK stands the test of time and is a classic shape. My mechanic drives an Aston and a Cobra and also has an old XK. He reckons the Aston is superb but that the Jag is not far behind in comparison of the cost factor. The Cobra - lots of fun if you can keep it on the road.
I would be interested to hear what you choose. Have you tought about the Bugatti?