As an AA Plat you'll be able to select seats on BA flights; "selection" on IB is an iffy thing - frequent "re-seating" stuff happens pretty often it seems.

OTOH the Plat bonus is a significant thing, even at a reduced earning rate; these are long flights.

OTOOH, my recollection is that the IB flight arrives quite a lot later than the BA flights, so if connections are planned, it might be a factor. I know BA is starting a morning flight to JNB (I'm on it in a couple of months) so that's another factor if you're flying Y - easier on the system IMO.

I would definitely see if my budget could handle WT+ however.