Normally at this time of year down south would be worse. But the forcasts suggest otherwise. I have never Sailed either Margaret River area or Albany so I can't give too much info. However I have spent quite a bit of time in Albany and there would be plenty of spots to windsurf especially if you are competant.
Albany and Margaret River are both beutiful places and there is plenty to do if there is no wind. If it was me i would head down south for a hoilday and take my kit with me but not be too disapointed if you go without a sail.
I believe the guy who runs Windsufing perth (Reg?) puts an event on down at Walpole/denmark way once a year. Maybe call his shop and he might be able to offer some better advice. Enjoy