Putting aside the FFP considerations, this is not an easy choice. UA has a decent seat but the food is awful and the service highly inconsistent. Also, UA crew will not keep the lavs clean, which can be a meaningful issue on a 16 hour flight. SQ has polished service, the food is generally very good, and they keep the lavs spotless throughout the flight. But the SQ seat is highly polarizing. I don't much like it, not because of the odd configuration, but because the bed is hard as a rock. UA is also easier to deal with if you need to make changes to your itinerary (particularly as 1K).
IMO NH is by far the best overall *A product on this route, but you have to connect in TYO.