I owned a diesel 2.5 TD5 discovery on an 02 plate, I now have a 4.6 hse P38 RR on a 97 plate. The TD5 returned about 27mpg on a run where the RR returns a max of 21.6 mpg so far. Saying that the RR only does around 9.6 mpg around town, the diesel TD5 did a lot more. The power on the RR is far better, you can notice it just trying to climb a curb!!, the diesel is less powerfull but you get much better mpg. If you intend to use your Land Rover everyday then the diesel is prob a better option, if on the other hand like me you use it as a second car then go for the V8 4.6 HSE, the performance in sports mode from 45mph to ......well in excess of the national speed limit is VERY good for such a heavy motor.

I bought my diesel RR because I wanted to travel in comfort with off road abilitys second to none
Since I purchased it with 105 000 mile on the clock 3 year ago I have put another 50 000 on it and been all through Europe and Turkey on the first trip and on the last trip which I had to cut short through no fault of the car I went through France , Germany Poland Latvia Lithuania Russia Kazakhstan Kyrgyzstan China and Mongolia then back to the UK.
The RR didnt let me down on those trips and at the moment is SORN but hope to be back and go back to Mongolia and carry on from where it had to be aborted
On these trips it seem to average about 30 mpg that calculated not the computer readings but around town and short trips about 25