Whichever one you take, I hope you will be in one of the planes that have been upgraded to the new First/Raffles/Economy class cabin. (2 SQ 744's were painted in the "Tropical" livery since late last Sep to launch the new service). Sorry I don't know which routes the upgraded planes are put on but I suspect they are the ones to LHR, JFK via FRA, EWR via SPL, SFO via SEL and HKG, and LAX via NRT and TPE.
All else being equal, I would choose the 777. On a full flight, the 744 will make you wait longer during boarding, deplaning, immigration, reclaiming baggage, etc. etc.
Enjoy your flight, and thanks for flying SQ!