RFA-1.
Xenadrine EFX gets returned like mad at my work (GNC), and ephedra is a far better fat burner than synephrine (aka citrus aurantium or however you spell it), which is the main thermogenic ingredient used in EFX.
The only reason I would take the EFX is if you can't actually take ephedra-based products after reviewing the Warning label, and possibly checking w/ a doctor.