It depends on what your core texts are. if the core text is a film, then pick the book; if your core text is a book, pick the film;
otherwise pick the one that:
- demonstrates the most competency with the different medium forms in terms of techniques; so for instance, written media vs. visual media/theatre have very different techniques and so it may be if you can pull it off, be able to compare them easily (I think of Frankenstein vs. Blade runner kind of matching works well, in terms of matching a written with a non-written form)
- is easier to analyse. it may be worth watching the film as well as reading the book, to make that assessment. Films generally are easier to comment on techniques especially in the film making process, such as music, lighting, shot angles, transitions, plot, character development etc.