First of all, tell you I play in challenging or hard so my options will probably not be good for higher difficulty levels.
Resources will limit the type of unit design. Being able of training a unique powerfull unit is frustrating. For that reason, I find more usefull in researching techs than increase unit numbers (groups, squad,...) as soon as possible, at least untill the 5 memebers/unit. That will give them lots of HP, and improve their survival. At the same time, I try to gather resources, that I will spend later, with the best weapon/armor techs.
Note: I will give horses to all units. If I have not, I'll buy horses untill I can find them. Wargs it the last option. Although they give init+dodge, I prefer rely in movement and attack 
- Cheap units. Basic component of my army. They take no resources. Full leather + spear, and later with the maze (15 or 16 damage). Traits: +HP, + 3 armour + 2 initiative. Once you get the 5 men per-unit tech, these units are still "quick" to train, low upkeep, high HP, deal lots of damage and loosing them in battle will not make you cry.
- Low Metal requeriment units. Once I access the longsword (around 20 damage), and better armor, I take the cheap unit and give them a longsword + a good shield. Rest in leather. For a little amount of metal, you get stronger units. At this point, they will benefit too with armor bonus (and essence bonus) in fortress. They are harder to kill, have counterattack, and anyway, loosing them is not a drama. Traits: the same. If initiative is high enough, can be changed by charge or bloodthirsty, but usually, init is never high enough
- Crystal. I generally lack this resource: easy to spend, hard to gather. If I have enough, I will use in trinkets +init +HP, but I usually save it for archers in the late game to give them +attack, or for some full armored unit.
These are my basic armies. When I arrive at the point of late units, bonus in fortress are so good that there is not much difference with giving them better items that will take a lot of resources. What's the difference between 25 armor or 40 armour, if they have around 100 HP, and increasing?. A lot of resources wasted. But up to you. Just try and decide.
In study: good players will probably tell you that the best is design units in a way that you can later upgrade their weapon and armour, with no initiative penality. Tuidjy showed in his playthrough a good combination: club man with the 2 strengh traits + ironskin, that can be upgraded to lightning hammer and better armor. I think it deserves to try. The lightning hammer is a weapon to have in mind, as deals damage that scales with the level.
