I've found the service to be almost the same on OZ vs SQ.
I've flown only on the A380s on each, so I can't comment on the seats in the 777. But I found that OZ seats to have a slightly better recline.
If I were you, I would check the fare classes and book the flight thay earns more miles on UA. In terms of scoring PQD, you're out of luck, unless you book a united flight with a codeshare, but those flights usually end up costing A LOT more.