Why not both? If you find any value in any book then, to me, it was worth it. As well, you may want to come back to a book later on after you have some experiences and a different perspective. I have not read "Doing to Done" so I can't talk intelligently about that particular book.
However, I would point out that you're on the official GTD forum so do take into account that the results/responses you get will be immensely skewed.
As for David's others written works, I would recommend them as well. Though it appears I am in somewhat of a minority with recommending "Making It All Work" after reading "Getting Things Done". I think "Making It All Work" is really for advanced GTD practitioners as it is, to me, "GTD In-Depth" and covers some topics in far more detail. GTD is tactics while MIAW is strategy. That's my perspective.
P.S.
Ready For Anything is also good too but feels more like an addendum or appendix to GTD proper. That's my thoughts at least!
Best of luck!