Have anyone use both of these before and can help me to make the decision, whether I should change my tool or not?
From what I gather from the website, iBooks Author is a Mac-specific eBook creation tool. It uses drag and drop functionality to help create eBooks for Mac and iPad. Looks cool, actually, but I'm not a Mac user, so I can't say.
On the other hand (and as KeithS already stated in his comment), Scrivener is a multi-purpose tool, which allows you to organize, write, and print or export your manuscript from scratch. It's capable of many things writers find useful during the creation process, and when that phase is completed, it has the ability to compile the manuscript into many different formats, for many different purposes, ebooks included.
I would disagree with the "word processor" assessment KeithS gave Scrivener (I'd describe it as a "writer's tool" personally, since it does so much more than the word processing aspect), but his analysis is accurate. They're really not the same type of tool.