Spencer - Mobile units are "professional" units that you have to purchase the officers and soldiers for on the direct trading market for GC. Mobile units can be upgraded higher than normal units and can be loaned to other countries. Extra soldiers and officers are needed to resupply lost troups as well so you will need a surplus of these hired guns. If you don't want to spend GC on officers and soldiers then build regular. Regular units should be the most common in a newer country. Focus on defense with interceptors and helicopter units first, lots of forts. Purchase high grade supplies and ammo and upgrade their fighting level to 450Qwith Weapon and ammo upgrades.