flying out of heathrow gives ther shorter 6 hour stop over, manchester is something like 18 hours, but that may change with the new stop planned.
We stayed over, singapore is a wonderful city amazing place, we went the night zoo, and spent the rest of the time sleeping and looking around...... tip..... catch a cab instead of walking around, its cheap and far to humid to be walking.
Singapore airlines is great, and even though we stopped over i did have bad jet lag for the first week, not great when doing job interviews! I have a feeling it could have been worse doing it all in one leg.
The parents use the transit hotel, and we checked in 11 hours early when leaving and spent that time in Changi airport (due to tropical storm). you have to do this, or consider this because the flight out is 1/4 to midnight and you check out of your city hotel in the morning. I think it is like a hotel on its own and can easily spend time in that place and travelling between the terminals on the train. Each terminal has something different in it. The map on the net does not show the sheer size of the place.