The Bible was also a total life guide, determining political and material decisions for millenia in many places, or at least being used so as a "smokescreen". That's the point, christianity as a whole no longer has any hold on the secular, material political world, or at least as much power as any other special interest group and less than most. Islam however is still seen as a blueprint for "proper" government and laws by the vast majority of it's practicioners.
Still, the religions are both abrahamic. They're more similar to each other than any other religion on the planet (barring judaism from which they both sprung). The history and context in which they both developped (especially post Renaissance and Enlightenment) is far more important than nitpicking at the content of their Holy Books, which are practically irrelevant if we're digging for fundamentally different approaches to politics.
