Definitely the Marriott in Singapore. I was just there in March this year. The Marriott is right in the center of the main drag, Orchard Road.
The Hilton is about may be 200 yards from the Marriott, but the subway station is right outside the Marriott. This is important for a climate like Singapore where it is hot and humid and afternoon down pours are always a possibility. All the major shopping malls are also connected via the subway entrance.
Did I mention the concierge lounge at the Marriott? Quality of food and service is probably one of the best.
Hope that helps and enjoy your visit.