I have not stayed at either of those hotels but thought I'd offer advise since I currently live in Singapore. As a tourist, you are better off at the Marriott (I actually live a couple of buildings from it) - you can get to a lot of places walking and its more convenient to public transport, etc.

You are better off with a taxi to/from the airport with three people. If you stay at the Marriott and wish to use public transport, I recommend the #36 bus (stops almost directly in front of the Marriott) rather than the MRT - the bus is direct whereas you need to switch trains 2 times albeit quite convenient to make the switch. The bus or MRT will cost less than SGD 2 for the trip to/from the airport. If staying for 5 days, I would get each of you an EZ-pass, which makes paying for public transport convenient and you can get a refund of any balance and the deposit for the card.

Someone mentioned traffic - frankly, thats hardly a concern in Singapore. The Orchard area does get busy, specially on weekends but there are ways around it.

Someone mentioned MRT express train - no such thing. There is only one subway/train system called the MRT and the line to the airport is a regular subway line. In fact, the trains in Singapore move rather slow relative to Hong Kong, Beijing, Tokyo, London, etc.