I'm not sure Lilith was really trying to set viewable pornography and reading erotic literature as a dichotomy.

For me there definitely is a difference however. When I read a story I want to be able to imagine every single detail, and a good erotic story allows me to that. I want to know exactly how hard the cock was or exactly how it felt to be touched, etc. But when view porn I don't necessarily need that much detail. Its more visibly titillating for me and I don't have to do as much imagining...make sense?