Priceless information, thanks for bumping this thread!!
I ran a few example with more than Fives to see what's the worth of boosting your defense (vs. health or protection). It usually speaks in favor of health/protection. Actual numbers vary depending on the character and the attacker's armor penetration but here are a few conclusions:
A defense mod set bonus (5%) is always worse than a health one (is equivalent to a boost of 1.5 - 4.0% of health value)
A defense main stat (+11.75%) is often better than a health main stat (+5.88%): More so for whoever has larger armor values to start with (mostly tanks), not so much for attackers
A defense main stat (+11.75) is always much worse than the protection one (+23.5%)
A defense% secondary stat (+1.5%) is almost always worse than a health one (+1.12%)
A defense plain value secondary stat (8 or 9) equates or exceeds a health one (300 - 400). In the case of Royal Guard, +10 defense = +650 HP.
Boosting defense is always more profitable for Tanks (larger initial values) than for Attackers
I can share the spreadsheet if anyone wants to investigate this further...