Have flown all 3 either in J or PY - some thoughts:
- Choice between JFK and EWR has been debated ad nauseam probably depends on where in NYC you're coming from more than anything else, but the lounge situation at JFK is pretty dire right now (Air India or Primeclass or $30 food voucher), while VS Clubhouse in EWR is decent (but windowless and gets a little crowded)
- Also if you don't have precheck JFK T4 security is a hot mess in the evening as well
- The nonstops can feel really long (especially when you're 11 hours in and realize you have 6 more to go), and if you have lounge access it can be nice to break up the journey with a shower in one of the FRA lounges, but I think I prefer not having to do the whole prepare cabin for landing - disembark - reboard - takeoff sequence with the connection in FRA
- Personally prefer the SQ23 schedule (late night / morning arrival) compared to SQ21 (morning departure / evening arrival) - on SQ21 the crew turns off all the lights after serving lunch, but if you're on EST I find it's a huge struggle to fall asleep, whereas on SQ23 I just sleep the whole way with breaks for food in between (and then nap after landing in SIN)
- If flying premium economy and traveling alone would prefer the nonstops on the ULR if you're willing to pay for the single seats at the back of the cabin (so much more space and nice to have no seatmate)