it would be worth it if the majority of the trips on cx are premium (1st or biz). aadvantage does have limits on coach class mileage (as least for usa members). but they also have the maximizer bonus which offers 10k bonus miles for 6 premium class one way trips.

also, aadvantage offers extra bonuses for premium class flights on the long hauls from us/canada. 20k one-way/50k roundtrip from yyz, 10k/20k for all other gateways. i picked up another 30k bonus on my rtw with this special.

at this point i think aa offers by far the most opportunties for bonus mileage earning activity. my 105k miles of flying earned me well over 300k miles with see the continents, special route bonuses, ttbns, etc. new route bonuses pop up all the time, even on existing routes. and all 1w status flights will give you bonus miles associated with your status level.

start it off right by singing up for the platinum challenge, and take it from there.