Jon Bon Jovi's the better technical singer; more range, more control of his voice, etc. But the strength of Cobain's voice was always in the amount of emotion it conveyed; anger, sorrow, what have you.
So it's more a question of preference: do you prefer a singer with range, vibrato and control, or a singer whose voice basically wears its heart on its sleeve? For me, it's a case of apples and oranges. It would make more sense to pit them against artists similar to themselves. Like Bon Jovi vs Joe Elliott, or Kurt Cobain vs Dave Grohl.