They're really not competing at this point. 7159 is the one spec to rule them all. ECMA-404 (which was kind of silly to start with[0]) isn't really relevant to most developers anymore.
ECMA-262 even explicitly uses 7159's predecessor (4627) with two exceptions[1], one of which is the top-level compatibility headache 7159 fixed, and the other just requires the API to disregard the "MAY" in section 4.
