Both films are political, both biographical and both thought provoking.
The Duchess starred Kiera Knightley as the 18th century aristocrat Georgiana, duchess of Devonshire, held hostage in a loveless marriage. She is unable to leave her husband's estate (which comes complete with a mistress in his bedroom), until she bears him a son. Once delivered, she is unable to leave because he threatens to keep her from ever seeing any of her children. Based on historical events, but of course interpreted for the times and highly hypothetical. Who is to say she didn't keep her lover Charles Grey, on the condition she remain discreet?
W was also based on interpretation of events. Not ever a Bush fan, I was surprised at how sympathetic a portrait Oliver Stone was able to render. I actually liked the W shown here, it gave me a whole different perspective on him as the son of a President and the leader of a nation. Although I haven't been converted to the Republican Party as a result, I think my point of view has been broadened.
Historians can argue the facts, but it's likley truth eludes us all, whether we are the biographer, the audience or even the original subject.