Can't speak for all airlines, but many only actually use "Brace Brace!" For a PREPARED emergency landing (in which case the brace position and meaning of the command are demonstrated to the pax during cabin preparation phase) If mentioned in the safety video then Brace may be used.
For an unprepared emergency, varying with individual airline SOP, the "brace brace" can be omitted for the reasons mentioned above. The pax just aren't really familiar enough with it in a sudden emergency.
Some variants I can recall from over the years:
Heads down, stay down!
Bend down, hold your knees!
Bend down, protect your head!
Bend down, stay down!
Brace brace, heads down!
Etc