I just went to Perth a couple of weeks ago, we flew Virgin there, and Qantas back. Virgin were lovely, but Qantas was HEAPS better. Bigger plane, staff were even lovelier and more accommodating and helpful. It was a much more pleasant experience, and the cost difference really wasn't that great. Totally worth it IMO.

As for car seats in rentals, make sure you question them on the phone. We used Europcar, and they were hopeless. The seats weren't fitted when we arrived, so we had to stand in the cold rain with the kids while the guy did it. He had no idea, I had to fix the seats! And I still wasn't 100% happy. Thanks for reminding me, I need to contact them about it, I can't believe they have someone fitting seats who isn't trained.