I've been doing my research on the 4.2 R's and I've also had a lengthy test drive in my neighbours, really good comfy car (even on the 20inch Senta's!) and it's the replacement for the Z I'm afraid lol. Can't say much for the XK but there along the same performance as your HR Zed, so you won't be getting much beside stunning looks and comfort, I'd look for the XKR if I were you. I've also had a ..chase shall we say with my neighbour to see the difference at full pelt and it certainly will still see you in the mirror (albeit TINY) but it is much faster and the grip around corners is beyond the Z, I tried my damn heart out lol.
If you can afford it go for the R
Much more compliant suspension set up, lot comfier than the Z
You will have to get used to the very light steering though, doesn't give you much confidence but it does turn well, you just need to get used to it.
Reason I'm looking...besides the obvious..the Z's clutch is just annoying on a day to day basis (and I've got the easier post 06' onelol)...The XKR drives wonderfully, makes you want to cruise and relax, but will still leave most for dead should you need to get a move on
Fuels not that much worse either.