The Thermal Bag is made from metalised polyester and is intended for use in emergency situations as a survival aid. Once used it is almost impossible to get back in the packet, the material used is noisy so will help keep you alive but don't expect to get much sleep. The polyester is very tough but once it starts to rip it can tear quite easily.
The survival Bivi is designed to be used in the same situations as the Thermal Bag with the added benefits of being lighter in weight, smaller in pack size, tougher rip resistant, quiet LDPE and can also be repacked and reused. The quietness of the material means it can also be used as a waterproof sleeping bag cover when a night out is not planned but quite probable.