For PvE certainly the necro.
In hard mode, the Elementalists damage output is pathetic. It doesn't even come close to what a Necromancer can dish out with Curses or Death Magic (because Necomancer skills ignore armor, while the Elementalists skills are all reduced to nothing by the high monster armor).
The Elementalist in hard mode is not a damage dealer, it's a support character. You can blind, knockdown, slow, daze. But for those effects, you do not need to be a primary Elementalist!
So go with Necromancer, be happy with your unlimited mana supply and use your awesome armor ignoring damage spells for nuking. And if you ever feel bored and want to know what you missed out on, buy some fire spells and watch how little damage they do. And get some earth spells and knock down enemies for fun

You can do that just fine as Necromancer since those conditions and knockdown do not depend on the skill level (e.g. they are just as good with 12 as they are with 16, only difference is some damage, which is too small to matter anyway in hard mode), while playing Necromancer as primary Elementalist is not that good due to the lack of Soul Reaping and since you will miss out on real (armor ignoring) damage then.
You could also consider a Mesmer. They, too, offer tons of armor ignoring damage and are great nukers. They got good energy management (Inspiration) and Fast Casting is an awesome primary attribute. Except for the minion master, any Necromancer style of play can be mirrored by the Mesmer, some even more effective, some a little less effective. As does the Necromancer, the Mesmer allows you to play any other caster class with great benefit from its primary attribute.
Personally, I prefer the Mesmer. Mainly because they look better