I believe a mixed bag of units either on the ground, in the air, or at sea, its always the best.
There is much more to A&A then just straight number crunching. Its better to keep your own units in positions and compositions where they have the most options, and can support each other the best. This forces your opponent to play more cautiously, and honestly, to protect themselves from attacks where they will lose significant IPC’S in unit value.
Think of it like chess, where controlling the centre of the board provides significant advantages because of unit mobility, and how controlling the check and pressure gives you the initiative. you almost force your opponent into moves they dont like, as oppossed to giving them free range.
anything you can do to make more oppurtunites, will make more headaches for your opponent. This alone provides them more oppurtunities to make mistakes you can capitalize on.
Artillery are part of a mixed unit game, because they provide oppurtunites to attack where you otherwise wouldn’t have any ( just consider china without thier fgt, and how useful art would be) the trick is to make sure you use your artillery as much as possible on the attack. because there is absolutely no point in having them as part of a defensive position.