I have both. The 90 is clearly better in terms of definition and focusing speed. Pictures made with it really stand out. But it has a big "con" vs. the 56, at least for me: Not being stabilized, as soon as light dims, you have to start raising ISO, losing the advantage gained with its better definition. You have not only a difference of 1,5 stops in terms of aperture but, on top, you have to shoot at a higher speed, which represents 2/3 of stop less light. Therefore, shooting hanheld, the XF 56 has an advantage of, at least, 2 stops of light.