Game of choice by viewing habits, perhaps - but certainly not by attendance.
I have no doubt there are more juniors playing AFL (sic) in Brisbane than in GWS. What would be interesting is the numbers for RL & soccer. My guess is that it would show there are less juniors playing all codes in western Sydney than Brisbane. Which means the field is ripe for the AFL.
By attendance Rugby League has had the western Sydney region to itself until this season. By participation Association Football wins hands down.
Using the FootballNSW 2011 Annual Report there are 203k registered football players in NSW. This does not include the region from the Hunter Valley to the Qld border who play in the NNSW Federation where there are another 50k registered players. The split up for the NSW Federation is roughly 165k registered players in Sydney and 40k registered players in country NSW. The Sydney figure can be split into 65k in western Sydney and 100k in the rest of Sydney. Of those playing competition football in Sydney 70% are under 21.
FootballQld do not appear to publish an Annual Report but from time to time you see figures quoted. They also freely give you the numbers if you ring them. The latest figures quoted that I read mentioned that the number of registered players in Qld had dropped 11% this season from a figure of 80k. This would mean that the number of registered players in the whole of Qld is about 10% higher than in western Sydney.
Comparisons between codes is a tricky business as the codes report differently. Association Football for example only report players registered for weekend club competition. AFL on the other hand includes participation in all programs but their 2011 Annual Report does break the total figure down into different categories. The total participation in all categories was 791k. The number of players involved in club competition across the entire country was 314k. To break this 314k figure up you have to look at small bar charts so the following numbers are give or take a bit. ACT/NSW just under 30k, NT just over 5k, Qld about 20k, SA about 50k, Tas about 10k, Vic 140k and WA just over 50k, Total 314k.
The equivalent number taken from Annual Reports in each case except for Qld for Association Football are ACT/NSW 18+203+50=271k, NT 2k, Qld approx 72k, SA 23k but there is a large rump that refuses to affiliate with the federation, Tas 12k, Vic 53k, and WA 37k, Total 470k.
I have found Rugby League stats hard to find. The Queensland Rugby League Annual Report 2011 indicates 16k senior competition players and 44k juniors across the state. The latest figures for Rugby League across the country showed 34k seniors and 120k juniors playing the game in 2008 with junior registrations roughly equal between the QRL, NSWRL and Country Rugby League.
From these figures it is clear your statement that all codes have less juniors playing in western Sydney than Brisbane is not correct with regard to Association Football, is probably not correct for Rugby League but is correct for AF.
Just as an indication of the task the AFL has "to re-educate" western Sydney, as Nines put it, the popularity of playing Association Football in western Sydney is 30% greater than the popularity of playing Australian Football in Victoria on a per capita basis.
That playing in club competitions does not necessarily reflect on attendances at professional sport is no clearer seen than figures I came across for the Gold Coast/Tweed region while trolling the net to get the information in this post. The region has 10k registered Association Football players, 8k registered Rugby League players and 2k registered AFL players. Attendances have been somewhat different.