There is a price difference, 316 is cheaper than 904 but 904 grade is more resistant to corrosive chemicals such as the acidic by-products from burning fuels that would typically congregate in a chimney flue.
They are both approved and listed by HETAS.
The 904 grade liner is held together with a Quattrolock seal - this is what increases it’s durability. That is why 904 is advised to be installed for a Multi-fuel stove.
904 has a higher PREN Value - A PREN value is the corrosion resistance rating.
If you intend on slumbering your appliance for long periods, burning wet wood, burning poorer quality coals (none of which is a good idea) or operating a boiler stove where the flue gas temperatures are cooler and condensation is more likely, or looking at your forever home then you may think at specifying a higher 904L grade chimney liner. In most instances a 316L liner conforming to BS EN1856-2, when properly cared for using correct fuels and best practice and regularly swept, will suffice.