Saag Paneer and Palak Paneer are nearly identical dishes, but Palak specifically means spinach. Saag Paneer may use spinach and/or other similar dark, leafy greens. Mustard greens are a traditional option for Saag, too, but less easily found in North American grocery stores.  In my Saag Paneer recipe, I have used all spinach, or any combination of spinach, kale, Swiss chard, the tops from a bunch of beets, and even a handful of broccolini. Every variation I've tried has been excellent. It turns out to be a great way to clear out the crisper drawer.