Having flown both SQ23/24 and SQ25/26 my personal preference is 25/26 because the stop in FRA helps break my sleep in a way that lessens the impact of jet lag upon arrival in SIN/JFK. Instead of a long sleep, I can only do a 2-hour nap on the sector between FRA and JFK, a shower in FRA helps. I can have a longer sleep later between FRA and SIN. For me personally, the extra flight hours and minor stopover annoyance are worth it because I lose less time due to jet lag upon arrival in SIN or JFK. Just my two cents. I do recognize that everyone's sleeping patterns or susceptibility to jet lag may not be similar.