A big petrol engine on lpg isn't necessarily cheaper to run than a diesel. In fact it definitely isn't if the diesel is being run on veg oil or some other alternative to pump diesel.
Depends on your priorities & circumstances.
Is boot-space or sill clerance important, what kind of driving do you do, short hops kill ecconomy anyway, but with warm up time can allow minimal time on gas, and then there's how available the stuff is around you or the routes you take. Basically the stuff can only offer a saving IF you can use it. BUT, my four penneth worth is here:-
I've got lpg on my 4.6 petrol and performance is completely unaffected. You end up getting less mpg on lpg though due to it's lower calorific value or something so it's not quite as cheap as it seems. Also there seems to be a much greater variation in the price of lpg than there is in petrol.
Actually finding petrol stations that sell it when on a trip can be a right buggr too. It would really help if those that did sell it actually bothered to stick signs up to say so (and show where the pump is!).
You can get overlays for your satnav to help, but I found on a recent trip to Pembrokeshire that none of the ones it directed me to actually existed.
Here at LandyZone we have plenty of very knowledgable members so if you have any questions about your Land Rover or just want to connect with other Landy owners, you're in the right place.