I would say Summer.
Winter is a time I don't like the cold-rain-and feeling yuck.
Summer is full of nice warm-hot days, sunshine, happy faces....
Not much to do really in Winter, unless going to the Snow?
Summer or winter it makes no difference to me, I take my holidays when I can get them. We are not beach goers, although we do enjoy a walk along the beach (on a cooler day). We were lucky enough to visit Coober Pedy, Uluru and Alice Springs last February when it was exceedingly hot, but opportunity doesn't knock twice. Despite the heat the trip was amazing.
We don't tend to just "go on holiday". We holiday with purpose! Our last holiday was a trip to Victoria - we had to go down to Ballarat for my grandparents wedding anniversary in January so we did Christmas down there as well. Once on the road we decided what route we would take depending on how we felt and how terrible my navigation was that day. Our next holiday will be to Brisbane in May as we both have work related conferences to attend. We will probably add a few extra days either side for catching up with family and site seeing. So we holiday as we need to. If I were going to go on a holiday just for the sake of going on a holiday I'd probably choose autumn in the Adelaide Hills - not too cold, but by no means hot and very very pretty.