You might want to read the article in our wiki
CBR and CBZ. The images themselves are in some format and that format can make a difference in the size and reading experience. Of course the wiki also contains articles on other formats. ePub would be the exact same images with an HTML document pointing to them. It might have a TOC that could be useful depending on the eBook.
I would suggest that you look at the tool you will be using to read them. Do you want bookmarks or other features, etc. the wiki article has some suggested reading apps.
Dale