Award availability will be the big issue with all ffp's-airlines
The AA ff miles & BA avios will not increase in value over time. Spend them now or watch them go to waste.
BA AA & IB are all Oneworld & ff partners, so nominally the same availability. But will vary.
Look at the AA sub forum
BA subforum
Your guide to spending Avios | 2020 edition
Some ffp's take a dim view on newbie's joining, transferring in credit card points and immediately getting premium cabin awards