In a nutshell, everyone goes vehicles because they travel the fastest and loot the most. However, using game theory, since most people use vehicles, some people use shooters to counter vehicles.
Vehicles, supposedly have an HP advantage in the late game and can only be countered with shooters using APC Parts. We’re not at the late game yet so take that piece of advice with a grain of salt.
That said, given that most of the upgrades in the late game involve fuel and iron, in hindsight, I partially regret going vehicles because they consume fuel. Had I gone with shooters instead, my food would be low (which isn’t really used for any upgrades) and my fuel and iron would be high, allowing myself to grow faster.
Regardless of the unit you pick, you have to focus on technology. That’s where the bulk of your troop attributes come from and applies to your whole APC.
Additionally, the military buildings such as the War Rooms, Garrison Hall, Garrison and Trap Room also provide buffs to specific units, defending units, and your Class APC respectively.