how big the land you're after? I'm building in Harrisdale and colleague are currently living in Piara Waters. they're relatively newer area and estates.
Harrisdale Green has government houses in the estate. not everyone would like to buy house in the estate which has goverment housing just in case it might affect the house price in the future. one of my colleague built the investment house in harrisdale green. I bought the one in the estate next to it. comparing to southern river, Harrisdale and Piara Waters are still a relatively new estate and a lot of people don't want to wait for all the facilities getting ready. so they chose the established community area/estate. I personally prefer Harrisdale/Piara Waters to Southern River.