As things are now, the difference between the units is by equipment alone.

You have the basic unit, and arm it with weapon, armor and equipment.

How about also choosing the unit type?

You'll start only with a "commoner", but in time (and tech learning), you might learn to arm barbarians, fighters, rogues etc.

The difference between them will be a minor stat improvement, and maybe a weapon specialty, but more importantly, it can add a special attack or ability for each unit type which will give a good boost to tactical combat.

For example:

Commoner - basic unit

Fighter - slightly improved stats

Barbarian - "berserk" ability in combar

Rogue - adds more gold after combat

Many other options can be added, and the unit type techs can be placed on different tech trees