Cotija is very dry, tends to be finer in texture, and is sharper because it's more aged. Think of it like grated Parmesan (dry, fine, sharp). Queso fresco is a fresher, soft cheese, not sharp at all. It's more like a crumbled feta, minus the boldness of flavor. I prefer cotija in this recipe because it has more flavor but use what you can find and prefer. If you can't find either, consider Parm or feta cheese, or omit the cheese.