Having borrowed an S Type for a couple of days I think I can give a bit more now.

I find my XJ8 superb. Aside from the niggles I have had it is really a superb car. Comfortable and great to drive, I could go on about its merits as i am sure other owners could too.
The S type was 2000 model 3 litre. Of course my XJ8 is superior.

But, this was a pretty good car too. Of course the engine did not pull like mine but it would be plenty for most people. The brakes were certainly not as good. But it was comfortable too and decent ones are available for £1,00 or under. Later ones are even better so I would say try both cars and see which feels right to you.