I have to say the fuel economy is little different between my old 300TDI auto and my TD5 auto.The TD5 has been mapped by Alive tuning and goes almost too well - that's about the only difference between them in terms of performance / economy. They both return about 23mpg locally,mixed towing use,then around 30mpg on a long run.
The 300TDI was better for grip off road though,the TD5 suffers from not having the centre diff lock.
Converting from auto to manual is possible,but hard work,a mate did it the other way round... Better to decide which you want in the first place.