You need more time in Australia. I'd spend an extra day in MEL at the expense of SYD but it isn't a big deal. The Sydney Westin is a fine hotel and has one of the liveliest bars/restaurants in town so I wouldn't worry about the age factor. It is less lively on the weekend. The W is not quite as good overall (surprisingly -- but also a tribute to the excellence of the Westin) but has a great water location, if you like a Marina. Not the best views (for that you have to stay at the Four Seasons or the Park Hyatt). In Melbourne the Westin is by far the best hotel in town, particularly if you get a corner room. All of the Australian starwoods have excellent concierges who can really make your trip more enjoyable (much better than in US hotels, and equal to the best in NYC). You can't go wrong with any of the starwoods in Australia, they are much higher quality than the US hotels and each has some special charm. Even the 4 points (which might be a Westin in the US) -- though it helps to be Platinum there, as that hotel was built in the Japanese style (the rooms vary in size by a factor of 3).