I think it depends on what you're looking for. The two sites serve different purposes. While book sharing does happen quite a bit on BookCrossing, the main goal of the site is for folks to leave their books "in the wild" and track their journeys. Paperback Book Swap's primary purpose is one to one book sharing.
However, I'm only a BC member, not a PBS member, so someone who actually belongs to both will likely have more info.
