mana shield or no?

ES is godly, if used correctly, like blck said. You can figure that by maxing ES and putting 16 points into Telekinesis, every point of damage you take is 1:1 ratio on Mana then. If you have 3k mana, you can roughly figure on having an extra 1950 HP from that (assuming 65% damage redirect from ES, too lazy to look up a real #). If you're a Lightning build, you don't even need to max ES, as your +skill and +Lit skills should have you covered.

there are 2 ways to building a lighting sorc, either maxing ES or not, if u do you'd be taking little damg but u still either way you need a huge mana pool if you are using ES at around 65%-95% absorb at 65 id say around 1.5k-2K mana with a moderate amount into TK at 95% id recommend 3k+ mana but the amounts of damg in PvP vary so much that a 40k+ hit could still wipe out a great portion of your mana, leaving you extremely weak as mana is you real life with ES, it dies you have to run and regen ur mana back and recast it