Mana shield has a defensive capacity all game. But I use terraptus to allow for more mana. This increases farming capacity and lets me farm faster and allow for a heal. One terraptus build Could be Flame breath, Holy light, Darkness, Terraptus. Mana shield cant really be used for gaining mana back but you can mesh it with Resore mana and a farm skill for a 3 way synergy. EX: Chain, Hex( or evasion, defend), resore mana, Mana shield. So, what do you choose? it all comes down to you play style. Both allow for fast farming and have the potential to be great ultimates in the early and late game. However Terraptus with alot of armor will be a late game power house over mana shield, unless you use defend and mana shield, which will come close but terraptus doesnt have to devote a defensive slot to an extreme late game skill.