If you are not familiar, both Goodreads and Readernaut are sites geared towards cataloging your books - what you've read, what you're reading, how far along you are, what you thought of said book, etc. etc. They (along with other sites like Shelfari) basically do the exact same thing. So, the choice of which one to use comes down to the little details. For my taste, Readernaut has a much more pleasant and usable UI/UX than Goodreads; meaning, I can actually easily put a book in my catalog without having to go through 5 confusing steps. However, recently Goodreads launched an app and included a Barcode Scanner, which makes cataloging books quite a bit easier.

Long story short, which do you use? Why? Does the user base alter your usage? Is it better to use a site where all your friends are (and, thus, get their recommendations and insight) or to use the site that makes more efficient use of your time? Or do you just simply use both?