Hi there and welcome to the forum
The Xf is a superb car but frankly not in the league of the A8 it's in the lower division IMHO
The Jag has a lovely interior if you get it spec'd right and the engine is very quiet for a diesel even today a few years or so in it is one of the smoothest and delivers a great deal of bang for the buck. I had the active suspension which makes it handle superb defies the size of the motor round the twisty bits. Expect mid 30's mpg for the s version I enjoyed mine but wouldn't want to run a big mileage in in to be honest it just doesn't feel that solid.
Where the Audi A8 shines is in the engineering ..it is totally over engineered in virtually every way and if you appreciate the way things are pu t together and operate you will appreciate the 8. The technology is way ahead of the jag and in today's world that matters, but having said that I don't think it's tech for tech sake it blends into the motor in a way which feels useable but still very special.
You also feel like you are driving something far more substantial that doesn't shout look at me like some other brands. The jaguar XJ is probably a better comparison to the 8. I did look at them before shelling out for my current D4. They offer good value for 1 to 2 years old, I was put off by the number of electrical and minor issues I read about and having to deal with the local dealers if I bought one as we had bumped heads before with my XK and the XF
Having suffered minor problems ranging from headlights which went off for no reason , rattles which drive me mad to even being stuck for two hours waiting for the breakdown chaps when the fuel pump gave way ....meaning trips to dealers and contact with the tossers who have no understanding of what it feels like to shell out a fortune for a car and suffer problems . I get stressed with cars which don't deliver as I have my own high standards and if I can avoid contact with dealers all the better as most of them are poor. I am not say Audi are better dealers but give me a reason to see less of them and I will be happy
hope this all makes sense
. Don't get me wrong I have a soft spot for the Jag and still have a 1996 x300 four litre sport which has been in the family for 15 years they do have a sense of occasion which many brands fail to capture
Oh and there's one other thing to consider, should we see the white stuff this winter
well park up your Jag BMW Merc and go build a snowman with the kids. I lost two weeks work with my Merc when we had the last bad winter. Previously my D3 sailed through the winter and did me proud
I couldn't even open the doors on the last Merc I had
Cheers
Glyn