For what it's worth, the local MMC dealer says the 4M40 is more reliable than the 4D56 - timing chain instead of belt. It seems to be a bit more powerful and use the same or less fuel. You can check http://www2.pajeroclub.com.au/forum/ and http://www.pocuk.com/forums/index.php for more info. My experience is that operating costs are similar. Diesels, generally speaking, are 20% more fuel efficient (In OZ they keep the cost of diesel 20% higher). The cost of everything on a diesel is higher - parts, filters etc. 4M40 parts will be hard to find in the U.S.
All that being said, I'd like to try the same thing, but with a manual transmission.