Mech units have Machine guns as well as a Bazooka, which enables them to be much more effective against vehicles such as Tanks, APC's, or Rockets. Though given the weight of the extra equipment their movement is 1 less than their Infantry brethren, only 2 spaces per turn. However, they aren't hindered by terrain, including Rivers and Mountains, and also receive a +3 Increase to their vision when on a Mountain in Fog of War.
Mechs also cost more to deploy than Infantry, costing 3,000G to deploy with the exception of Advance Wars: Days of Ruin, which has Mechs costing 2,500G to deploy.
|Mech (Advance Wars)|
While Mechs have stronger firepower, they don't move far, so utilizing APC's and T Copters will be very useful for getting them around in a hurry. Also, protect your Mechs--while their firepower is equivalent to a Tank, their defense is equal to Infantry, so be wary.
Mechs can also be spammed similar to Infantry with increased firepower, but less movement range.