Hi Paul,
I've owned 3 manual Disco 1's in the past (200Tdi, 300Tdi and 3.9V8) and currently have a 300Tdi auto. Each of these vehicles has been both our family car and my weekend toy and were/are virtually unmodified as a result.
From my own perspective I prefer the auto for road driving. It is less effort to drive even if, like me, you use the lock-down feature of the box rather than just sticking it in "D" all the time. I suffer from arthritis in my ankles so not having to continually depress a clutch gives me an easier life when stuck in traffic jams. Performance is much the same between the manual and auto 300Tdi's I've owned by the manual was definitely better on fuel (low 30's compared with low/mid 20's mpg).
Off-tarmac I think the manual box gives a slight advantage due to better engine braking. Having said that, locking the box in 1st provides good retardation. For slippery conditions I find the auto better as it is less prone to inducing wheel spin. I did Soar y Mynydd and Strata Florida a few weeks ago and didn't have any problems ~ although the water was pretty deep in places :dance:
Overall I would probably go for another automatic if I was looking to to change the current Discovery.
Hope that helps :lol: