You have to use ruin II if you predict a movement (like titan hard's weight of the land or landslide) or that if you have instant cast spells that needs to be casted in between ruin II that can't be casted otherwise when casting ruin I, ie. fester, aetherflor, spur, rouse, bane, enkindle. other than that, it is much much more efficient to cast ruin I.