There are some cases where BA miles can be better value if wanting to redeem domestically in the USA.

If wanting to redeem on longer distance journeys, I would suggest that AA miles tend to be better value plus, of course, no fuel fines to pay as long as avoid redeeming on BA. e.g. for JFK-SYD would cost 150k avios plus taxes plus fines vs 72.5k plus taxes ( no fines )

You need to look at the earning rates and the redemption rates for where you want to redeem to work the best value

There is also a better chance of attaining status with AA if travelling economy over BA , however if travelling in premium cabins, status with BA is a lot easier to attain than AA

I wouldn't use the credit card as a key reason to use BA since there are credit cards that credit to AA and sometimes have v high opening bonuses.