I just read your reply and realised that when you say LAX - SIN, you don't mean the SQ19 the non-stop flight, but SQ29, which is LAX-TPE-SIN. Or am I reading it all wrong? *confused*

If comparing SIN-NRT-LAX and LAX-TPE-SIN, I would choose SIN-NRT-LAX, because of the upper deck.

Hope this helps!