Used to do KUL/SIN-LHR a fair bit a couple of years ago. I think both overall products are comparable.

SQ new Y does have the edge with the new entertainment systems - bigger screens, more movies, TV shows, music. Service-wise, SQ tends to be more consistent but I've had absolutely fantastic service on MH from start to end and flights where the crew just didn't seem to care. Seat-wise, SQ's are a bit hard and they don't seem to recline as much as the older ones. The seats do translate forwards when you recline the seat. On MH, 34" pitch is 34" no matter how you cut it.

Food-wise, I think it may depend on individual tastes. MH usually has a spicy Asian option which I prefer - the briyani ex-LHR was great the last time. SQ tends to have a more bland chicken & potatoes or fish & rice. But I've been told there is also now a third Indian meal choice on SQ. But whatever it is, nothing beats the real thing on the ground (in Singapore and Malaysia where good food is cheap and abundant) so I wouldn't fret over it unless you have specific dietary needs.