OP: when will you spend your lounge time on Sunday? Both SQ & BR lounges are open once or twice a day, for 2-3 hours at a time, depending on their own flight departure times.
BR has a daily flight leaving between 00:00-01:00, and an afternoon flight that operates 3-4 times a week leaving at around 16:30.
SQ has a daily flight to HKG leaving between 00:00-01:00 and the new n/s to SIN leaving at around 09:30. (Something like that.)
Check their flight schedules on your date to see if their opening hours will coincide with your schedule. Both lounges have better catering.