It's hard to say ...
INXS was around for almost 20 years before Michael died. Michael by that time had really established himself as one of the great songwriters.
Kurt Cobain killed himself only about three or four years after Nirvana's first "real" success (the release of Nevermind). What was released was very good. But, I don't think he was around long enough to say whether or not he was a consistently good songrwiter over time.
So ... I guess I would say Kurt because we'll never know how truly good a songwriter he could have been.
Here's an even tougher question ... who was the bigger loss, Kurt Cobain or Jeff Buckley?