I like LibraryThing because of the reviews and seeing what is suggested reading related to a book I like. That's usually how I decide what book to read next. But I do prefer GoodReads, for some reason it seems simpler. A big plus for GR is they have a mobile site. If LT does I haven't discovered it yet. I like updating my reading stats-where I am in my book, etc, from my phone.
I am way more organized on GoodReads, again, it just seems simpler. But I don't get as many useful recommendations from GoodReads as LT.
So, they are both good, I usually toggle between the two when I'm online.