Letting the PKK take on the IS is a much better option in the sense that like many other militias in the area, who have either been taking on each other or the ruling regime in the state they inhabit, they see the IS as a cancer that needs to be taken out. The United States should simply provide air cover and try to clear a pathway to move the civilians caught in the crossfire out of harm’s way.
A final note to make is that there is no military solution to dealing with ISIS, there’s only a political one, and that can come only with a unified political voice from the Iraqi people themselves. The Iraqis of all stripes (Sunni, Shi’ite, Kurd, etc.) should feel that they have a voice in their own governance. This was a direct failure of Nouri al-Maliki.