When measuring a band's level of influence, two things must be taken into account: What sort of impact that band had on other musicians, and how that band affected what music fans listen to.
No matter what anybody thinks about Nirvana or Metallica, whether one happens to hold a favorable or unfavorable opinion of either or of both, it's an undeniable fact that Nirvana has been far and away the more influential of the two.
Metallica was, in its heyday, one of the best American Heavy Metal bands of all time. They had the respect and admiration of legions of fans around the world as well as that of their contemporaries. And in later years, their slowed down, softer, subdued sound demonstrated that it was possible to write sissy songs and continue to brand oneself as metal so long as one has already made a name for oneself in metal.
But Nirvana changed rock and roll forever. They spawned countless imitators. They may not have created a genre, but they were definitely progenitors in it, and they were the face of grunge and alternative rock. Metallica never achieved that level of fame, notoriety or influence.
Are the musicians who played with Metallica more talented than those who played in Nirvana? Absolutely. But being more talented doesn't mean you're going to be more influential.
Hands down, Nirvana completely blows Metallica out of the water in terms of being influential. Anybody who lived through the 80s and 90s who's still around to see where the chips have fallen can see that in a heartbeat. It's not even a contest, it's a clear victory for Nirvana on that front.