They are essentially the same thing, as they are both ways of saving webpages for later viewing. However, they serve different purposes (at least IMO).
Bookmarks are for quickly accessing sites you frequently visit (for me, some examples are facebook.com and gawker.com)
Reading List is for saving a specific article or listing for later reading (where as gawker.com is one of my Bookmarks, I would use Reading List to save a specific Gawker article that I wanted to read later).
Really you can use them however you want, this is just what works for me. Hope that helps.