As said above, Vox and Nagafen both have 32,000 hp. Nagafen can triple for about 225-255 while vox is around 200. Personally, I'd have to say Nagafen is a lot harder.
Nagafen: 32,000 hp, triple for 255, AoE spell Lava Breath ( 700-800 damage ), and what makes him the hardest is that friggin Mass Fear spell DragonRoar.
Vox:32,000 hp, around 200 a hit, AoE 500 damage ice spell. Yes she can Complete Heal, BUT it's easy to avoid unless your casters are oom. Have one full group of casters start a chain stun sequence when vox is at about 30% health before she starts healing. And whoever said she can't be stunned anymore either doesn't know what's goin on or has some really horrible casters. Trick is to lower resists first, easy to chain stun around the end. Have your group of casters stun, while about 5 or 6 wizzies Ice Commet the *****.
Nagafen is harder cuz of that mass fear spell, sends everbody running while nagafen can just summon you all back and lava breath you all to death and claw you for 225 until you're nothing but bloody pulp.