Nothing complicated.
18s+:
Thunder -> Aero -> Aero II -> Stone II.
12s-:
Aero II -> Stone II.
AoE:
Holy.
What matters is 90% of cases is Damage per GCD. Duration doesn't come into play since most fights will last 18s+ and since the time between each DoT is only 6s, it is actually pretty pointless to talk about durations.
