The value of health and defense are dependent upon each other.
If your toon already has high health/prot, you'll get the most value from defense.
If your toon already has high defense, you'll get the most value from health/prot.
If the two are balanced, then you should keep them balanced.
Not very helpful, I know. Damage mitigation is very nuanced and difficult to boil down. To really understand it, you have to consider linear vs non-linear effects (like +100 health vs +1% health), the health-value of defense (5% damage reduction ~ 5% health increase), the order of operations and how stacking works (do two 100% increases total to a 300% increase or a 200% increase?), risk tolerance (is 5% damage reduction better or worse than 5% dodge?), expected healing (the more you expect to heal, the greater your effective health, and the greater the effect of defense), and character specific nuance (like the Stormtrooper's % increase to defense).
In an attempt to be helpful:
Toukai's suggestion was good :)
Especially if you have a healer in your squad.
If you don't have a healer, then focus solely on protection (health secondary) instead.
