Nice comparison. The difference in their depreciation is huge. The golf despite being around 25% more expensive to begin with will be worth over 10% more than than the Astra in 3 years time! I wonder how the gtd compares to the rest of its typical rivals on total depreciation over 3 years - any residual value geeks out there? 
I think the only depreciation indicator you can bank on is GFV as a % of RRP for standrad spec (options are poor value for money from a residual values point of view and options are personal and, well optional. Absolute minimum the car company will give you - a show of them "putting their money where there mouth is".
At 56% the GTD is hard to beat.
I suppose the main condenders would be BMW 120D (184), Astra bi-turbo (192), A class A220 AMG Sport (170), A3 2.0TDI S-line(184)
A-class A220 AMG Sport retains approx 51% (reverse engineering the deposit, monthly payments and interest to determine the GFV)
BMW 120D M-sport retains approx 46%
A3 S-line 2.0TDI (184) retains approx 54%
Astra/focus/Megane are all around 35-37%
GTD is the smart money's choice.