Wizards are much more one-dimensional, focused almost purely on DPS (aside from a few 'utility' abilities such as porting - which druids also have)
Druid is basically "jack-of-all-trades - master-of-most". If you prefer to be a DPS, go with Wizard. Otherwise, Druid is the call - among the top 3 solo/farmer in the game. Top 3 healer, top 3 buffer, utility spells out the wazoo - best root, best snare, best DoTs as far as damage to mana ratio.
Of course, Druid also gets SoW which can't be underestimated
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