If a textbook has highlighting software on it such that you can highlight certain parts of the text then already they are just as easy to take notes on as normal textbooks. Also, since you oftentimes do not want to write on textbooks so that you can get money back it would be easier to take notes on the online version.
20 years ago, a person never would have believed that we would be able to take notes and do millions of things on a portable device with a touchscreen. Here we are today, tablets and smartphones are seen everywhere. I believe that tablets are easy for note-taking with because of styluses and other tools used to help writing. There are actually specific pens that can be purchased that work with just tablets. No more wasting papers, use a tablet.
I personally prefer to use a textbook for note-taking rather than a tablet. For me, it's easier to keep track of where I am in a book than it is on a screen. I tend to have a harder time keeping track of my place on tablet, with a book you can use a bookmark to keep track of where you are.
Unfortunately, there is not an application that perfectly organizes your notes to the point that you could easily take notes. Paper and pen and the traditional methods are always the best for making notes. You easily divide your notes and go back to where you need to for any sort of reference.