Damage calculator (browse to the tab)
I created a general damage calculator which accepts a variety of modifiers including HP, ATK, DEF, and DMG (reduction), and outputs the modified damage and--most importantly--how this affects the hits til death (HTD). Hits to death is a stat I came up with that's a bit more intuitive than effective HP, but serves the same purpose. For example, imagine a scenario where you could reduce incoming damage by 90%. 5% more damage reduction on top of that seems paltry, but it actually DOUBLES your survivability.
One notable result is that defense down amplifies damage far more potently than an equivalent percentage of character attack up. 50% attack up increases your damage by <50% (because it's additive with assault skill). Defense down acts as a direct multiplier; 50% defense down translates to a 100% damage increase. Furthermore, the benefit increases as you near the cap of 50%: 49->50% defense down provides a nearly 4% damage increase!
Defense down is thus extremely valuable, and reaching the cap is quite difficult, even in a raid situation. Most teams will max at -40% (20% each from type A and type B debuffs), which provides a 67% damage increase. You can use the calculator I've provided to calculate how much your team would benefit by switching in different skills, especially on your soul.