I've never been to the Gold Coast, so I can't comment on that, but Rockingham/Mandurah are nice areas. With the freeway into Perth, they now have a pretty easy commute. They're also close to the WA wine country down in Margaret River. There are lots of parks, everything from playgrounds to larger wildlife reserve parks very close by. Rottnest Island is a ferry ride away. Kings Park up in Perth. There are wineries up in the Swan Valley as well. I love the areas around Perth.
When we move over, if we end up in WA, which is where my wife's family is from, we'll likely end up somewhere around there.
Hello - my husband and I are looking to take the plunge in Jan with our three daughters. We have visas for Perth and like you was looking in the Mandurah/rockingham areas. Possibly Quinns Rocks? Hoow have you got on? I'm finding it all a bit daunting!!!