The Harbourside Marriott is better located for most aspects tourist wise, close to the habour, and convinient to train, bus and ferry for getting around to see any sights of interest. The area it's in is mostly business though, so there's not a lot of nightlife directly around it, but it's a short walk to the Rocks area if you need more activity.

The downtown Marriott is a bit further removed from the main tourist icons, but still not that far away. It looks out over Hyde Park, and is just around the corner from Oxford Street, which has a large gay scene associated with it and is a more lively sort of place at night (though it also has many decent restaurants to dine at as well).