Aluminum or tin foil has a shiny and matte side as a result of the production process, but the difference between the two is negligble when cooking and storing food.
During the final step of production of aluminum foil, two sheets are rolled together at the same time to prevent breakage due to tension. When the two sheets are separated, the inner surfaces are dull, and the outer are shiny. Differences in cooking performance are so small they require instrumentation to measure.
Shiny surfaces absorb and emit less radiation, but we don't use aluminum foil when cooking with radiation (like a microwave) because microwaves create currents in metals.
