I must admit, I was biased against Library Thing, seeing as how I am already frighteningly involved in Shelfari. However, at first glance, I have to admit that Library Thing may be a better program -- it's faster to load, clearly organized and the forums are much easier to follow. That being said, Shelfari is, well, pretty. On Shelfari, you can arrange books on your virtual shelf by date read, reading progress, favorites or manually shuffle them around to your heart's content. It appeals to the manically ordered part of me. Library Thing seems a bit more utilitarian.
Any Library Thing followers want to convert me?
Image courtesy of the Fighting Librarian.