The Quill editor replaces the core editor. The core editor however is needed for the image upload extension, so you can't use both at the same time.
I'm not a friend of the Quill editor. After using it for a while, you can't go back to the core editor without having to delete the posts written with the Quill editor. The code saved in the database with the Quill editor is totally different and posts written with the Quill editor will render as rubbish in the core editor (and I guess vice versa).