Just to throw some more options (confusion?) into the mix, you might want to look into the SIN-LAX non-stop (with its executive Y) and then connect to SFO. You'll arrive at LAX Bradley Terminal
and then transfer to another terminal for the flight to SFO. (e.g. UA in T6, 7 & 8).
But I have heard and read a lot of favorable comments about SQ exec Y on the non-stop. It has much better seat comfort than regular Y - 7 across instead of 9 or 10 plus more 3-4 inches more seat pitch. It does cost a little bit more, though, but nothing like the premium for biz class.