Never go full wizard.
To be honest the -50% cast time is not worth the lack of skills from Cryo or Pyro.
Here is my honest answer,
I have a level 81 elementalist, so I have just dabbled into the class.
Crymancer:
Immense burst potential in electrocute alone.
To my knowledge, you must have gone Cryo2 in order to obtain the damage boost attribute however.
Great control of the battle field when paired with Elementalist Petrify.
More time to cast Meteor (C2) and Freezing Sphere (C2)
Pyromancer:
the best wizard class for Damage over time
Being a DoT focused class, Elementalist brings in more DoT skills in the form of Hail and Frost cloud (C3)
Pyro will never fall-off because INT adds damage onto the ability at a flat rate; this means 100 INT = 100 extra damage each tick (If my understanding of stats is correct).
Elementalist:
A nice utility of almost every element
Attributes allow for resistances and raw damage output
Recommend at least getting 1 point in Petrify, and maxing out Electrocute or or Meteor.
Because of Rain (C3), Both paths are easily doable.
Recommendations:
As much as Elementalist combos with Cryomancer, the requirement to go Cryo2 puts it at an odd spot. Limiting your damage potential with defensive abilities like Gust and Subzero Shield, As long as you’re going for pure burst, this is the way to go.
Personally I would prefer Pyro2 into Elementalist because of the overall AoE damage. Fireball + Flame Ground + Flame Pillar into Frost Cloud/Hail + Meteor/Electrocute already give you enough damage output and can be prolonged across a longer battle, as long as you have the SP for it.
Going Pyro2, your Flame Ground will always be available (Duration = CD) and your Fireballs will apply a Burn DoT as well.
Its up to you, but I prefer overall damage output rather than a huge burst