I would sooner change gear than gearboxes
Expensive things them Auto boxes and they take the edge off the MPG generally 5-10 mpg which is likely to leave you where you started
As a previous owner of both a manual and auto CDTis I can offer some genuine experience rather than the generic statement above....
Both the CDTis I owned were ZTs in 135ps tune. In terms of raw performance the manual is quicker and marginally more economically, but both factors are dependant on your driving style and the sorts of journeys you undertake. On average I achieved 39mpg in the auto and 45mpg with manual, but with a heavier bias to stop-start urban driving conditions in the auto.
The main killer with the auto is the extra VED cost due to the heavier CO2 emissions, but then if costs are your priority then the 75 is not the car for you. Instead you can enjoy an easier and more refined driving experience (not to mention more effective off-the-line starts).
The auto is the more reliable transmission of out of the two. While the gearbox is robust, the clutch and its hydraulics are not, and replacement of the dual-mass flywheel (if required) is very expensive. The auto is generally reliable if looked after with timely fluid changes done properly. Gearchange solenoids can fail but as these are mounted externally, they are not too difficult or expensive to replace.
If I was buying a CDTi again I would choose the auto. The slight deficit in MPG and performance are easily offset by the more refined driving experience.