It's pretty rare that I read an actual book, but when I do I prefer to do it on my phone. It just works better for me. My wife will have an audio book, physical book, and a book on her Kindle all going at the same time. I don't know how she keeps them all straight!
If you use Audible and a Kindle, Amazon actually syncs ebook and audio book up for you, making it easy to go back and forth. Of course that wouldn't help much for physical books, though.
No, we don't pay for the service like that. I think she does all of her audio books through the local library using an app called libby or some other one.