I thought Jimi was better overall.
Dont think you can really split it up into strictly "better songwriter," you have to take what they gave us.
Nirvana never really did anything for me, i think they just tapped into a certain amount of anger, or frustration that i never really got, but Jimi holy s**t! The Band of Gypsies album is phenomenal, so is Electric Lady Land, then theres all the singles, amazing body of work.
Would disagree strongly that Kurt wrote more good songs, then again, that is of course subjective! I would put Jimi's cover of All ALong the Watchtower above nearly anything that Nirvana wrote, for all the same reasons that people like Nirvana too.