Elem/Mesmer or Mesmer/Elem?

Which of these is a better overall build? What's better fast casting or energy storage? Do you really run out of energy that often with an elementalist?

I would say Ele/Me because the enery storage comes in real handy.

Several elementalist spells cost 25 energy and give you Exhaustion (a temporary -10 max energy). With Energy Storage, you can handle that. For a Me/E, 25 energy is at least half your mana pool. I'd say that unless you've got a specific build that really needs Fast Casting, go E/Me.

Fast Casting ranks far below energy storage in my thought, because Fast Casting, while it allows you to shoot off several spells in a short period, means you will be sitting for a while with no spells to cast due to blowing all your mana. In most cases it's not that huge a difference - I'd rather have the mana to cast my 15-25 energy spells in succession, allthough slower, than to fire off Meteor Storm in 1.3 seconds then have nothing to do while my mana recharges.