AA, BA and IB have a "joint business agreement" making it easier to manage their flights more seamlessly. I have an AA-marketed BA-operated flight whose seat selection I can initiate from my reservation at AA.com itself. Unlike old days I don't have to call anyone, don't need to know BA record locator, etc.,

I would suggest you book whichever flights are cheapest or make most sense for you and then figure out seat selection. Whether something seems "easy" is often a matter of familiarity, and there is always 1st time to learn anything, but it should not be too difficult.