How many miles do you do a year roughly?
About 12k
12k is quite low these days, and with the current fuel prices you might struggle to make difference back, this all depends on how much more the TDI costs over the 1.8T
regardless of the amount of miles you do, the TDI will hold its value for longer than the petrol, personally i'd go for the DERV, however if i was to buy an anniversary i wouldn't do any mods as IMO the anniversaries should be kept as they are.