Should I purchase new tins or plastic refills for chocolate powder?

It matters little, While it takes more energy to make a tin than a bag, HOW much cocoa mix are you buying to justify. It takes 6 megajoules to make an aluminum can, Tin with a lower melting point, slightly less. It takes less than half a megajoule of energy to make a plastic bag.

On the other hand, buy cocoa powder in reusable container that sells it by weight?

Tins ultimately are more sustainable than any soft plastic packaging not only from a recycling point of view but even otherwise. They can be reused safely and for longer periods of time. In fact the only time you will need to throw away a tin is when it is accidentally crushed beyond a certain point where it cannot be restored to the previous shape and that too is just a point of aesthetics unless of course there are holes in your tin which is the only case which can be justified as a practical reason to throw them away and if you will, that too can be solved by applying a small amount of tin metal on the hole to cover it up.