Soloing very weak mobs: Speed Surge (Gonna steamroll 'em anyway, might as well do it faster)

Soloing medium to hard mobs: Life Surge (Helps accuracy, and better life leech help survivability)

Partying against weak mobs: Either one

Partying against hard mobs: Life Surge (life leech on speed surge is minimal, the evade down on life surge is more helpful)