I'm running the R380 and 300Tdi. It feels very Land Rover-ish. It is almost too slow, almost too loud, almost not enough power, but somehow still manages to be operational. I have been over the mountains here several times in both mild and in hot weather. If I don't have idiot drivers in front of me, I can do the passes at 4500 feet or so without going below 50mph. Hills above a certain length or steepness just slow it down, no matter what altitude I'm at. If I go above 5000 feet (I've been as high as 6500 in 80 degrees ambient temps) I start to notice how gutless it feels even on flat. I would like to drive a D90 with the same set up someday to see what the difference is like.
That last point about the vehicle aging well is probably a good one. I have a '65 Series IIa that is almost completely original, and when I compare it to the Defender, I have to admit that the Defender is becoming a classic vehicle in the same way that the Series is. In the next 10-20 years, I don't want to be dealing with parts issues for an engine that is any more obscure than the 300Tdi. If it fails, I will probably rebuild it, or get something like an LS replacement. I might look into the Td5, but only if there are enough of them around at that time to justify the change.