We dont see as much of the older XJ's anymore, BUT ide go with the Audi
XJ's can be reliable, but when they are not they're very very expensive to repair!!
XJ's very smooth to drive, i used to love cruizing in them, even the 3.2 V8 (the most reliable)
If you can push it, i'de go for the new shape XJ6 (X350), the 3.0 V6. Very nice motor, loads of room and very reliable!!! Don't know what they fetch, but these days i bet their cheap!!!
BUT saying all that about the Jag, Audi's aren't perfect, they can also be expensive to maintain. I'de drive both, then decide! You might get lucky with a nice XJ and you might get unlucky with an Audi
Dan