What metallised film is
Metallised PET or BOPP is an ordinary plastic film with a very thin layer of aluminium deposited on it in a vacuum chamber. The metal layer is a tiny fraction of a micron thick, yet it sharply cuts the oxygen, moisture, and light that pass through the film.
Because the base is still a plastic film, it bends and folds without cracking, and it costs far less than foil.
What aluminium foil is
Foil in a laminate is a continuous sheet of aluminium, typically a few microns thick. Intact foil is close to a complete barrier to oxygen, moisture, light, and aroma.
Its weakness is handling. Repeated folding can crack it and thin foil can carry pinholes, so it is laminated between films that protect it and the structure is matched to how the pack will be formed and handled.
Choosing between them
Metallised film is the usual choice for snacks, biscuits, and confectionery, where a good barrier and a long enough shelf life matter more than an absolute one.
Foil earns its cost for products that lose value quickly in air or light, or that need a long shelf life: coffee, tea, milk powder, some pharmaceuticals, and export packs.
A quick rule of thumb
If the product needs months of protection and isn't very sensitive to oxygen, start with metallised film. If it is aroma-, oxygen-, or light-sensitive, or the shelf life is long, specify foil. Share the product, target shelf life, and packing machine with your supplier and they can confirm the structure.