As a tourist I have been to both.
I liked Perth better...but be prepared to drive. Perths is a very clean city, that did not have much to do within the city. We visitied the Pinnacles (4 horus north, Wave wrock 5 hours east, Maragaret River 4 hours south, Tree Walk, 2 hours south of Maragaret River), Freeport 30 minutes away. I wish we had more days to drive to Monkey Mia and further west into the Gold Towns.
In Sydney is a very nice CITY were you have 2-3 days of attractions w/o driving.