Larry Zerner, a copyright attorney in LA, has laid out very clearly why you want to copyright your screenplay, not register it with the WGA.
The problem with the Script Registry is that many writers are using it as a substitute for registration with the U.S. Copyright Office. As a result, in the event that their work is infringed, the writer will almost certainly lose thousands of dollars. And, in many cases, a writer who only registered with the Script Registry will be precluded from filing a lawsuit because the economic realities of litigation.
Read the rest on his blog.