I've always felt that LibraryThing is more about cataloguing your books (it's more... librarian-like) and Goodreads is more about the social side of reading (with reading status updates and all that).
Yes, LibraryThing is not free, but you also don't get ads. And I also think that the GR interface is not very good.
LibraryThing has less reviews... but you also get to avoid all of the crazy/annoying gif reviews that plague certain books on GR (3 examples:
50 shades /
Clockwork Princess /
Walking Disaster).
Anyway, it depends on your personal preferences.