I am not very familiar with Land Rovers, but one thing to bear in mind is that with either of those engines you will be limited to using the Isuzu 5-speed diesel transmission (or automatic version of same, if you are so inclined) unless you are willing/able to fabricate your own adapter system. This means that the rear diff must be centered and the front diff must be offset to the passenger side (on a LHD vehicle). Don't know whether this is Rover-friendly or not.

That said, the 2.8l 4JB1-T(C) engines are considered one of the best light-duty truck diesels around, from what I've read. The 4JG2 series is larger, at 3.1l (I believe) but is not direct-injection like the 4JB1 series. I think it is not as highly regarded. You will have to browse around some UK or Australia/New Zealand forums to get more information, as those are markets where people 1) speak English and 2) were able to buy vehicles with these engines.

See the link in my signature, I've got a forum link on my site you might try.