AA would be the best one to use in terms of the value of the earned miles (remember that 1 mile in BA/QF/AA/IB/AY/LA/CX is worth a different amount in each plan, with up to 300% divergence depending on usage). AA with the Platinum challenge would start earning 100% status bonus as well as lounge access, and good for future flights until Feb 2008. So be sure to sign up for the Plat challenge ahead of time (this is presuming that 10K miles worth of flights are done in a 90 day period).