I believe any army unit is made up of 10,000 individuals.
A Militia is 10,000 untrained people, levied hurriedly and given whatever weapons and items are at hand.
A Regular Army is just that. One Regular Army is 10,000 individuals who are properly trained, well disciplined, and outfitted with weapons and armor.
An Elite Army is composed of roughly 10,000 innately powerful beings who are also well trained, disciplined, well outfitted, and who work together as a team. What does innately powerful mean? They could be moderately high hit dice monsters. They could be powerful beings like the githyanki. They could have numerous innate powers. They could be regular ordinary people who have been magically altered to have special powers (they all regenerate as trolls, they can all fly, they are all Stoneskinned, they all have Darkvision, they all have magical armor and magical weapons, and so on.)
An Epic Army is just that. Roughly 10,000 individuals with very great powers, working together as an army. They all have very great power, natural or augmented, powerful magical items, powerful spells, or are very powerful monsters teamed together.
Based on what I read in the rules, a Regular Army will slaughter Militia easily, unless the Militia have some advantage such as being inside a fortified city.
Likewise, Elite Armies will slaughter Regular Armies.
Epic Armies, will slaughter Elite Armies.
But ...
Militia cost nothing to build.
Regular Armies cost 1 point (see the rules for details.)
Elite Armies cost 10 points.
Epic Armies cost 100 points, and you must have elite armies ready to upgrade to epic first.
So, you must make a choice: If you are spending 100 points on armies, do you build 100 Regular Armies, or 10 Elite Armies, or upgrade a single Elite Army to Epic?
The choice, is up to you!
Just remember there are 20 other powers facing your problem also. What to do? What to do? ...
And incidentally ... you might think you could build endless armies of Militia, but remember there is a limit (roughly equal to your IC value, see the rules) to have many armies you can have activated in any given Turn.