For the first 3 books, Love Route is more focused in story and character development while Slave Route is more focused on the sex. Save for book 3, which I'm pretty sure we get some story scenes I did not see when playing the love route.
Book 1 it is mostly just a change in interactions, kinda same as Book 3, but in book 2 in Slave Route you are a generic person that Azula takes a liking to and in Love Route you are Zuko.
But book 4, they are VERY different stories. In Slave Route you are taking over Tenzin's body at the start of season 1, while Love Route you are an empty body while Kora is recovering around ending of 3, start of 4. Love is also still unfinished.