I would go manual every time, especially for a diesel, and particularly if you are using off-road. Plus you will get at least 10% better MPG.
Don't get me wrong, I love the V8 auto's, but the diesels in auto form (personally) I don't think are that great. Modern (as in the last 2/3 years) diesels work ok with an auto box, but not older ones.
Matt.