I personally would lean towards spell speed. Right now, its hard to get enough spell speed to get very good returns (10-15 spell speed being less than 0.01sec increase), but Piety seems to be poor as well. From what little I've tested, each piety seems to give around 8 MP @50. From personal experience, more people have died to my spell not landing in time compared to people dieing from me being out of MP.