Well, you would never get heavy anti air for the soft attack. Even art1 blows it out of the water:
1 batallion of Artillery 1 (+first improvement, you can get just as fast):
cost: 126
SA: +32.4
1 batallion of Heavy Anti Air :
Cost: 200 (+58%)
SA : +6.7 (-79%)
I could see against CAS, but I've noticed in early SP battles the air game is insignificant.
I never considered the hardness factor. It's interesting. But early SP game, the AI is pretty absent with hard units too..
I've made 40W Marine Divisions that most basic infantry couldn't pierce by using just one HSPAA battalion. It was as Japan so I probably used some XP to increase armor a bit, but I can't remember. SPArt delivers more punch, but it's also more expensive.