This is a fascinating question, made all the more interesting because I disagree with it. I assume that you would explain "preference" by saying that the text makes clear Austen thinks favourably about the strong female characters in this book. Whilst I think you are perfectly correct about Elizabeth Bennet and the way that the author obviously shows a preference for her, I would disagree with the presentation of other strong female characters, most notably Miss Caroline Bingley and Lady Catherine de Bourgh, who are clearly strong female characters but also not presented favourably in the text. Consider, for example, the way she tries to turn Mr. Darcy against Lizzie and thereby advance her own position:
She often tried to provoke Darcy into disliking her guest by talking of their supposed marriage, and planning his happiness in such an alliance.
Clearly such mean behaviour does not indicate that a character such as Caroline Bingley has a firm place in the affections of Jane Austen. In addition, consider the way that Lady Catherine de Bourgh takes Lizzie to task for the supposed alliance that exists between her and her nephew. Both of these strong ladies are therefore not shown in the most flattering of lights. Lastly, you might like to consider the way in which female characters who are not strong are presented in a better light than these strong female characters, such as Jane and Georgiana Darcy.