The truthful answer
Six hours can work, but it is not a free-for-all city day. Your real usable time starts after disembarkation, immigration, baggage decisions, transport out, transport back, and security before boarding.
If you are nervous, tired, carrying bags, or arriving during peak traffic or heavy rain, Jewel or Changi Airport is the better recommendation.
Best use of a 6 hour window
Pick one compact target: Jewel only, Marina Bay only, or Chinatown/Maxwell only. Do not chain multiple city districts unless your immigration clearance is fast and your onward gate time is comfortable.
Changi's official Free Singapore Tour may also fit some layovers from 5.5 to 24 hours, but you need to follow its registration rules and remain in the transit area before joining.
When not to leave
Do not leave if your passport or visa status is uncertain, your bags are not through-checked, your next boarding time is close, or you would feel stressed rushing back.
LayoverSG's bias is simple: when timing is unsafe, stay at Changi and enjoy Jewel properly.
Next practical step
Compare 4h and 8h layover plans