For second graders, I didn't see as many title choices as I did for a high school student or an adult/college student. I also use e-storia from Scholastic for my son. They offered a set of free books when I signed up. The rest of the books are not free, however, the books are reasonable and support his classroom teacher.
I like the options available in iBooks rather than ePub. The ePub books are black ink on white "paper" and are difficult for me personally to see. I prefer to have a dark background with lighter print. Besides the background choice, other advantages of using an e-reader include more choices of titles. Some people, especially my husband, read more than one book at a time. While traveling it is much easier to carry one e-reader than a stack of books. You can also highlight, make notes and change the font size. Using the contents allows you to quickly look back to another chapter. When using the bookmark, you don't have to worry about losing your place. My son's favorite thing at my mom's is to pick up her book and close it, thus losing her place.
Annotations on a text can be cumbersome. You know you saw a quote you wanted to go back to, but now you can't remember where you saw it, you spilled your coffee on your notes and page 5 is now
missing. Using the notes, highlighting features and bookmark can keep you organized to complete your task.