Support guns (I mean all of them) have a bit less attack and piercing and a bit worse stats overall but do not effect the division in any negative way, like paratroopers can still paradrop and so on, just pure gain.
Ahhh very good callout, hadn't thought about it that way.
Support guns are suited for small divisions because the gain from the guns are big for a small division and the gain from other support companies who compete with the support spots are low for small divisions.
Got it, small divisions benefit from support guns more than they do from other support companies.
For a large division you are probably better off using the support slots for the support companies and use line guns as they are known as (those that take up combat slots).
You can only use one of each support gun but you can use as many line guns as you can fit which again is more useful for large divisions.
Thanks, that explanation as well as the statistics others posted helps make it much more clear as to
why you would use Support versus Line anti-air, which was my original (poorly worded) question.