Can we really gauge fans based on 9 Sydney teams against one cities clubs?
If you're gauging interest in the sports as a whole then yes, comparing the average attendance of all the clubs from each sport against each other and the population of the city and it'd give you an interesting number and a gauge of the popularity of each sport in the city, by it's self it's not enough of a gauge to determine which sport is more popular in the city, but still an interesting peace of data I'd say.
Doing it club by club however is ultimately pretty useless though as it's easy to manipulate the end result, for example comparing the two highest drawing clubs from each sport against each other would just be stupid because though the highest drawing club in sport A may out draw the highest drawing club in sport B by 50%, if you add the attendance of all the clubs in the city together sport B may out draw sport A, which would suggest that despite the fact that sport A has the biggest individual club in the city sport B is in fact the more popular of the two sports, of course this number alone wouldn't be enough to prove that sport B is indeed the more popular of the two but it'd still be a very useful piece of data in discovering which sport is truly the most popular of the two.