I thought that both Ostwind and AA flak emplacement could be merged in one ability/slot like Heavy Air defences for example, but if this is impossible then yes Ostwind is a better trade.
I think so too, it would be good if it could be done this way, otherwise I'd like to see Luftwaffe Ground Forces keeping their defensive utility and have the tasteless Valiant Assault Infantry buff replaced with an Ostwind (this would solve the excessively good synergy of VA and Falls too).