If you like the surly discomfort and undistinguished, pudgy-looking frumpishness of the 911, go for it; I'm sure it's faster because a friend of mine had one while I had my 5.3 V12 XJS. If you want timeless class, artisitically gorgeous style, comfort, quiet and performance, the XJS wins hands down. The XJS is also plenty fast, and amazingly, it seems to accelerate at the same velocity at 90 to 120 as at 60 to 90. And it makes a wonderful sound! Plus, if you want to get somewhere in comfort, the XJS wins hands down - and my 911 friend agreed. Guess which car gets more favorable comments and thumbs-up stares, and more attention at car shows? The XJS by a landslide! After '91, they were also very reliable as well.
Porsche sucks. Jaguar is very nice compared to a Volkswagen, I mean a Porsche. If want to rule the highway, buy a Corvette ZR1, it will kick the sh#t out of any Porsche in the same price range. Look it up, Corvette even beat them at their race track in Germany. Don't get me wrong, Germany makes good cars, but to compare Jag XJS to a ugly 911 is a shame.
I own a 1980's 911 and my father has an 1980's XJS. I've driven both. The 911 has better initial acceleration, but the XJS is very strong above about 70mph. It also feels much more stable and hunkered down. In a low speed course, the 911 would probably be the better car, but for high speed and certainly grand touring, the XJS is sublime.
I own a 1988 Jaguar XJ-SC V12 5.0 and I raced my buddy in his 1987 911. I buried him, it was a highway race, he had me off the line but in a matter of seconds he was far back in my rear view.
I've also raced my buddy in his 1989 Corvette ZR1 and it was a neck and neck race until we hit 110 MPH, it seemed like he maxed out but I kept climbing.
The Jag is a beautiful car, it is much faster than any other car mentioned here. Plus it is far more classy. :D.