The "original" Marriott on Hyde Park is well placed for lots of things depending on what you want to do. It is on Oxford Street - the gay centre of Sydney, with stacks of restaurants etc. It it across Hyde Park from Sheraton On The Park, but it is admittedly a little bit out of the tourist beat unless Oxford Street is what you are here for.
The Circular Quay one on Pitt
STREET, is near the Opera House, the Bridge, and historic 'The Rocks' shopping and restaurant era etc.
The Circular Quay train station very close by is where to catch a train to domestic airport, altho a cab is only about double train fare, which is ridiculously pricey by world standards, and is heavily under patronized as a result. 2 persons is a slowish train is not a great option to Airport.