Having recently flown on both in F, the qualitative difference in seats & service is immaterial. I would therefore make the decision based upon other factors including mileage needs and schedules.

I travel several times a year on this route and most often choose to go in Raffles (J) on the nonstop flight. 15.5 eastbound scheduled hours (usually closer to 13.5 actual) versus 17.5 hours plus the hassel factor of connecting at Narita makes SQ19/20 a compelling choice. The connection at Narita really ruins the whole experience of SG F.