Another value of Avios over Advantage Miles is that BAs booking engine allows you to get access to CX (and other inventory) about 2 weeks earlier than AA does. For long haul premium awards, that is a real value where you want multiple seats. I just booked a block of 5 seats that will not be available to an AA member because the flights didn't yet appear available for booking on AA. I'd rather pay a great number of miles and some surcharges to know I get early access to these.