Moopla,
They are both good.
There is probably a little more for the kids to do at Neurum creek, with the bike track and other activities organized by the owners from time to time. Neurum creek can be quite busy at times, though and feels a little more commercially focussed.
I personally prefer sandy creek. It is not as big, a little less crowded and a lot more relaxed.
In fact, we are off there this weekend with a large group of families from my kids school. The group has booked the whole place out. I think web have about 25 families going.
Praying for sunshine
Pete