Personally, I think that attending lectures is better.
First of all, personal contact of a student and his teacher gives the student better understanding. There are a lot of nonverbal things that influence learning that we can not ignore. Once, I was reading a book on Russian history by Kostomarov and had some questions. When my teacher at school gave a lecture about this topic I understood everything much better.
Second, you can ask questions about the things you do not understand. At the lecture about Peter the Great I could not understand a few things. I questioned my teacher about this and everything became clearer. Teachers also summarize large volumes of material and do not let you get lost in mountains of details in books.
So that’s why I think that attending lectures is more important.