“Foil balloon” describes a family of laminated films, not one universal material. Two common constructions are Nylon + PE + aluminum layer and PET + PE + aluminum layer.
Nylon-based laminate
Nylon generally provides better toughness, strength, gas tightness and softness. Those properties can make the finished balloon more tolerant of forming, handling and demanding inflation use. The tradeoff is higher material cost.
PET-based laminate
PET-based laminate can be a practical, cost-conscious choice for many products when the design, size and inflation conditions have been validated. Performance should be confirmed with samples rather than inferred from material name alone.
Which should a buyer choose?
For larger balloons and helium-filled products, nylon is more often recommended because strength, flexibility and gas retention become more important. For smaller or air-filled products, PET may be suitable after testing. Final selection should consider dimensions, seal design, valve, expected inflation duration, distribution conditions and target price.