The better the unit, the longer it takes to build, but if you have enough territories, you may be able to build a good unit faster than your opponent can build a poor one. The better a unit the better its intelligence, speed, armour, weapon range, weapon damage and fire rate. If you choose to change production to a different unit type, you lose all the time invested in the scrapped unit, so it pays to be decisive. The action hots up around the blue enemy base. Mini-battles are great fun.
Snipers make the best drivers though as they're the most intelligent infantry.
The intelligence of a unit helps it act better all round, including being better at dodging attacks and shooting up the bad guys. Units are quite capable of acting sensibly of their own accord - for example if a nearby vehicle is empty a grunt will get in it without you telling him to. The computer AI is pretty good. As far as I can tell it doesn't cheat - you can watch the factory production timers and check the units. It will attack relentlessly, and often in a more coordinated fashion than other games.