As the OP, now that my travel finished, I'm just reporting back that I was happy with my choice to fly UA nonstop (SFO-SIN) on the outbound and SQ nonstop for the return. I slept well on both flights and appreciated the much-superior soft product on SQ. I wasn't too bothered by the off-to-the-side foot well on the SQ flight, though I can definitely understand why people would be. Flying SQ home saved about $1200 (although it was OPM, I could actually apply the savings to my hotel room in Singapore; so it still was a benefit to me); it cost me about $2500 PQPs (which I don't need for this year) and some 40,000 RDMs. I was willing to pay that to try SQ.

But honestly, I was happy with both flights. I would make a similar choice in the future, assuming the PQPs didn't matter. If they did, I would probably have stuck with UA both ways.

Thanks to everyone on FT who expressed views, including especially the recommendation to check one-ways...which saved about $2500 on this itinerary. In the weeks before I booked, nonstop business roundtrips were pricing at $16K or $17K (!) (I wouldn't have been flying the nonstop, or I wouldn't have been in J at all.). At the time I booked, they (briefly?) appeared to have fallen to something like $9,500, as I recall. Booking as two one-ways ended up costing about $7K.