I also received a question about whether both implementations guaranteed unique. I asked Steve to follow up with MS to get a 'good' answer, and Paul Randal from the storage team was kind enough to provide this answer:

"These two intrinsics are thread-safe, and multiple concurrent users will get different UUIDs. http://blogs.msdn.com/sqlprogrammability/archive/2006/03/23/559061.aspx gives more detail on this topic."