It depends on your usage, but generally, cast iron tawas are best to use. Still, let me share some key points to consider when using an Iron tawa a
Iron Tawa is heavier as compared to Aluminum tawa. So if you need a lighter tawa, one should use the Aluminum one.
Iron tawa, if not handled with care, can start rusting.
Aluminum tawa will get scratches easily as compared to Iron counterparts.
Some claims of aluminum utensils, in general, have particles of aluminum in food.