It has changed from patch to patch, but honestly? These days lightning mage doesn't use shields at all other than the one he gets from blocking (he has a psijic ability on both bars).
Before this last patch dropped, he was using Hardened. As of the recent patch, I switched him to Empowered for several reasons. One, longer duration. Two, more reasonable cost. Three, magicka regen bonuses.
But, like I said, I don't even have room for a shield on my bar anymore. I was one slot short of abilities I wanted to slot as it was, and then they remove major sorcery from the sorcerer (seriously, wut?), so bye bye shield guess I'm doing entropy instead (and I'd rather slot streak than entropy, because entropy is kind of boring by comparison).