One thing is clear at least.
Firstly, it doesn’t matter how sustainable the packaging it. It matters far more how sustainable the contents of the package are.
For example for a plastic refill package, I find it very hard to believe the plastic would weigh more than 10 grams. 10 grams of plastic correspond to 20-30 grams of CO2 emissions.
If there’s half a kilogram of chocolate powder there, it also creates about 2-3 g/g of CO2 emissions. But because half a kilogram is so much more than 10 grams, the ecological footprint of the contents is about 50 times more than the ecological footprint of the packaging.
The metal packaging probably weighs far more than the plastic packaging: I estimate it’s not 10 grams but 100 grams. Steelmaking has emissions of 1-2 g/g, so the steel packaging would create 100-200 grams of CO2 emissions. That’s about six times more than for the plastic packaging, but steel recycling is easier than for plastic, so by recycling you can reduce the emissions. I suspect if the steel is recycled enough many times, that the metal packaging would win over the plastic packaging. I’m not sure about this though: winning would need that recycled steel production causes 16.66% or less of the emissions that producing steel from iron ore causes. I failed to find a source for the emissions of recycled steel.
However, for average recycling rates plastic would win. Wikipedia says only 65% of cans are recycled.
