Officially the answer is the AirNZ lounge, in that that's the one they will direct you to at check-in.
Historically it was also the best option, if only because they had a UA staff member there (and on most days, not just any staff member, but
Annie!) - however that is no longer the case.
AirNZ will still have the advantage that they announce the UA flights, where SQ do not. Otherwise, as others have said, it's basically personal preference.