The books and online courses cover basically the same materials.
However, if you want to practice writing and reading, the books would be better for you.
They are helpful because there are questions that you have to answer and an answer key in the back of each lesson. It is a good writing practice. There are more exercises in the books than online actually.
As for online lessons, you can practice listening and do the lesson quizzes. You can also ask questions to the teachers and other members.
It is all up to you, kjade808-san :)
It's almost identical to be honest. I got the book a few weeks ago, and I just found the site today, but the lessons I've looked at are pretty much word for word with the book. So I don't think it's necessary to get the book as well unless you're more of a hands-on learner like me. Good luck! :))