The main advantage is that HHVM is available now, PHPNG won't be stable until later this year at best. For my code base HHVM isn't compatible so we're eagerly awaiting the stable release of NG but damned if we didn't try to upgrade to HHVM.
EDIT: I didn't think I would need to mention the obvious reason nhtechie there mentioned. That was my whole point after all, for the ones who understood it before downvoting, but whatever.