I believe a soldier's chance to kill is calculated by this formula:
Code:
chance_to_kill = dlf * const * lethality * 1.1 ^ (attack-defense)
where
dlf is the difficulty level factor (believed to be 1 for normal, 1.5 for hard, 2 for very hard)
const is some constant which's value is still a matter of debate
defense is the sum of all applicable defense values (eg only armour for rear missile attacks, armour+shield+skill for frontal melee etc)

This can tell you how much better a unit becomes when you change one of it's stats...

Consulting the big heads listed above isn't a bad idea at all...