It's a bit of a silly dichotomy. No linguist will deny that certain sentences are ungrammatical ("cat sees I"), so they are just wrong. And lots of philosophers believe that normativity, what is wrong and right, is an important part of language. But it appears to be something dynamic, language changes over time, yet I can't change language by pretending dog means cat today.
So prescriptivism is wrong in the sense that you can't claim to have the sole authority on something. But descriptivism is vacuous, in that it doesn't explain the pervasive tendency for people to consider things to lie on continuum of wrong and right.