At Changi
Use it for dawn departures and for arrivals so late that Marina Bay would be a closed kitchen and a dark walk.
Ask whether the room is in the transit area or in the public area. A landside hotel still means you have entered Singapore.
SIN · Where to sleep · published · checked
Where to sleep on a Singapore layover
The train is good enough that a city hotel is usually right on 24 hours, unless the onward flight leaves before the first useful train. Then sleep at the airport, and check which side of immigration the room is on.
Marina Bay
The postcard base, and a simple taxi or train back toward the airport. You step out into the gardens and the river.
You are paying for the view and the location. A quiet room matters more than a club lounge on a one-night layover.
Civic district or Chinatown
The right base on 48 or 72 hours if you want breakfast near the food. Check the walk to an MRT station before you book. Ten minutes is fine. A bus puzzle with a suitcase is not.