Just as an FYI, most times a syncopal episode is self correcting. Pretty soon after a patient becomes horizontal on the floor they'll start regaining conciousness as blood flow becomes normal in the head again.
If your pt is uncincious long enough to make you consider an airway adjunct, there's probably something more going on than just a syncopal episode.
As for what to do, do the least invasive technique tgat works. If all you need to maintain latency is a head tilt/chin lift, then that's all you need to do.