NRT is a ghost town and you won’t be able to enjoy it much if you transfer, plus it will add a few hours to your trip.

I’d go for UA direct on this routing. The miles earned are a huge plus as well as the PQP towards status. UA’s business class is not the best but the seat beats SQ’s pathetic offering by a mile. The least comfortable flight in biz I had recently was on SQ. Their seat looks nice but it is anything but comfortable to sit/sleep in for such a long flight.