Well, it depends.
Both materials have merits and disadvantages. For instance, metal pans are better at conducting heat. They also distribute the heat more evenly in the lasagna. However, some metals are not safe for cooking as they react with the food.
Glass is unreactive. However, it takes a long period to get heated. It also does not cool down quickly. If you are looking to keep the lasagna warm, you would be better off with a glass model.
As you can see, each material offers advantages and disadvantages. You have to choose which one you’d prefer.