Given the choice between UA and SQ, I'd go with UA without a second thought.

Yes, SQ is a much better experience than UA, especially in Y, however it's a significantly longer flight. eg, SYD-LAX-JFK is just under 10k miles. AKL-LAX-JFK (AirNZ/UA) is 9k. AKL-SFO-JFK is a little under 15k miles, and around 30 hours of flying time (plus at least a few hours in transit).

If it was in J or better it'd be a much more difficult decision...