Welcome to FT Etxebas, official welcomes along shortly I'm sure.
I would largely agree with your assessment, a couple of additional points to consider:
T4S (BA) pro : better lounge
T4 (IB) pro : much better for domestic / Schengen connections
BA pro / IB con (if booking via BA) : BA online & app superior for seat selection, check-in etc IME
I do prefer IB in business class for that route these days. Must say that I hadn't noticed better legroom in BA ET vs IB mainline Y, but will take your word for it.
Presumably the A330/340 flights on IB arrive/depart T4S though?