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standard infantry gets that buff as well…

In pretty much every game I play, mechanised is inferior to standard.

That said I have a tendency to ignore bonuses altogether and just churn out militia, because it's cheap. Which I generally get away with in Comintern.
Honestly the cost factor can be as important as the buffs. Particularly in an emergency, the best unit is the one I can actually produce without selling every other resource off on the stock market, and then get to the front line before it's too late.

Lv.3 milita is one of the best units in the game.

Agreed. Saved my skin on multiple occasions.

L4 is actually like 60% better for 30% extra cost. I appreciate that I'm lucky to be an old player and did my dues to get blueprints long ago, but it is the truth, and the BEST blueprint unit you can have as a player.

As to the mech vs mot inf debate: it depends on SO much factors! What is your resource situation, and do you have pre-existing production facilities? Do you need to churn out units quickly, or do you have time to build a huge army in reserve? What kind of armor type is your opponent using; mostly unarmored, light or heavy armored? What is your play style, are you on 24/7 or just once a day? All that needs to be considered when making a choice... just remember that you make ONE choice, and the worst thing you can do is develop them both a little bit. Make up your mind for every game you play, and then stick to it.

(This is true for other unit types as well; when you decide AC will be your rusher, stop researching and/or building LT, and the other way around).

True, but that comes in days later. For the early game though, lv.3 is crazy strong.