We just stayed at the Conrad Centennial....fantastic hotel; especially as a Cat 5. We liked the location. It was nice to be away from Orchard Road. Great executive floor with nice lounge. Breakfast and dinner were both wonderful in restaurant downstairs. The hotel arranged a private guide and driver to pick us up at ship, take us on a tour, and then deposit us at hotel. As we took the "Spirit of Singapore" tour the day before through the ship, we just had her take us to the places we hadn't seen.

The Marina location is nice. Birdseye view of the fountain and light show. Short walk to Marina Mall to do a little shopping.

If you want to see more reviews of both the Conrad and the Hilton, go to www.flyertalk.com and go to the Hilton board. There is a recent thread reviewing both hotels.

edited to add:

Here is the url of the Singapore Hilton thread on flyertalk.