Perseverance vs. Insight
give me one good reason i would want increased base health rgeneration when im already far in kill range for the enemy, vs 15% reduced cooldown on my summoners (especially 
).
the health regen doesnt even persist until youre relatively safe again. when it turns off at 25% health youre still very much threatened and are not gonna stick around in lane or anywhere near an opponent to benefit from it.
not to mention, you can buy regeneration and defense if you need it in form of items, but the only item that provides summoner spell cooldown reduction is 
imo the mastery needs a buff to take effect below 40% health instead of below 25%. it should at least help you sustain you back to a point where youre not finished off by 1 ability.
right now there is 0 competition between those 2 masteries. summoner spell cooldowns win hands down every time, unless youre heimer maybe.
since the mastery is in the defensive tree, maybe it could do the same as meditation and regenerate a % of missing health instead.
look up his base health regen. and darius is pretty flash reliant and has in combat sustain built into his kit. he doesnt need it.
. You can use it to let yourself get away with healing your ADC more. And if it only getting you back to 25% HP isn't enough, then use some charges of your refillable potion. Or just play very safe in lane for a while, since she can get away with doing that in a lot of situations.