I have found the 2010 Switchblade 9m to be a good all round kite for Perth's varing wind conditions. I weigh 80kgs and it's lighter wind performance 15-18knots is quite impressive, whilst still managing up to around 25knts before things start to get a bit uncomfortable. So at your weigh you should have no problems! I have previously owned Crossbows but found them to be slighty slower in turning speed etc (admitadly they were older models ie 2008.. so maybe the newer models are better??). My 2010 Switchy is up for sale in the 'buy and sell' section at the moment due to an upgrade to an 2011 kite. I'm located in Peth as well, so your more than welcome to take it out for a test run if you are interested

for normal perth i'd try a 10 and a 7 switchy.
i've got 2011 11m and an 8 atm, but the 11 is new this year so ya cant get em 2nd hand. i'm 72kg but am concentrating on the kiting for the lighter winds with a surfboard right now.
from all reports the 7m from 2010 is an absolute weapon!
I used the 8, 10 and 12 2010 SB heaps when i was in Greece and really enjoyed the 8 and 10, but spent more time on the 12 due to the wind having heaps less power than it does here.
i cant comment on the crossbows but it depends on your style of riding and what ya want the kite to do. AFIK th SB is a real all terrain grunty beast.