Dumb Question (Banner of Command)

When should I, as a support, should buy Banner of Command instead of Locket of the Iron Solari ?

I recently played a game as a support ( ) and we were against a full ad comp. When I was about to buy Solari, I read the stats for Banner and thought that since it had more armor than the other aeigis alternative, I decided to purchase it instead. Of course its active was worthless because their team can kill the minion quickly, so I thought why does Banner have more armor than Solari? I mean, Banner's active is more against AP comps than ADs, so why does it has more armor. I think that Solari is way better than Banner due to its incredible active, but I feel that the armor and magic resist should be switched.

BTW, has +30 armor, +60 magic resist, and its shield active has +60 armor, +30 magic resist, 10% cooldown, 400 mana, and the promote active.