Been to both places and did it with luxury (nothing below 4 star, didn’t compromise on cheap flights). If you have kids - Bali is boring unless your family loves water sports, relaxing on beach / resort life. At best you can stay a few nights at Ubud / then spend a few nights in seminayak. We also stayed at Jimbaran (Ayana resorts) - popular Rock bar .
Singapore is great fun for all - be it universal, sentosa island, Singapore zoo, great nightlife, food, casino, pubs and shopping at orchard
Stayed at Jen by Shangri La and Holiday Inn clark quay.
Singapore I used my credit card everywhere . Bali did carry a bit of cash.
Avoid Dec for rains.