During the cooler months I'd certainly go back to the Westin as the outside area is lovely, but at this time of the year Yalumba is the better option. BTW the Westin brunch covers the three lower ground restaurants I prefer t be seated in the Hunters Room and grill rather than the huge Blue Orange but def prefer outside seating, weather permitting.
Been to both but Yalumba wins hands down for an all girl party atmosphere. Westin is more classy and better location, but for fun I'd certainly be heading to Yalumba. Enjoy whichever you decide to do.