Sounds like United is the best choice for you (but to maximise your benefits you'll need to fly via SYD/MEL to get to/from USA). UA has million mile lifetime status (equivalent to lifetime PLT on AA) but this is based only on UA metal.
Once UA rejoins the *A-wide mileage upgrade scheme you'll be able to use UA miles to upgrade on NZ flights - although availability is extremely limited especially on flights to/from USA.
NZ airpoints may be a bad choice as cheap economy fares earn little or nothing, and status is really geared to full economy and business fares.