Honestly, I'd use nether if I was allies doctrine.
Mech Inf. IS very good on all fronts, and lights tanks are also good but lack in cities and bad at defence. So regarding your question, mechanized infantry is better. But, the allies docrine has a speed debuff. THats hinders the "Blitzkreig" tactics I like to employ. The fast, Mechanized units are good with Pan-Asian and Axis, but in allies they suffer. I reccomend, if you are allies doctrine, build a lot of planes, and keep constantly upgrading them. Allies is built for reaserch and upgrading. If you continually improve your units, you should be able to stay ahead of most players.

The Allies Doctrine? My preffered tactic just uses planes to attack, while infantry protects them. BUt if you want one, I reccommend Motorized or Mechanized Infantry.

use mech infantry

good aa value , it can fight against light armor and infantry and good option to spend food

you can use them as attacker and defender

best army seems for allies doctrine like that mech infantry , sp artillery , tank destroyer , planes and ships
but you will need a lot of oil and iron
so I think you can keep with infantry and artillery to save some oil and iron instead of mechanized and SP
infantry , artillery , tank destroyer , planes and ships

bomb with ships , planes and artillery
and move your infantry with tank destroyer to where you bomb