Por que no los dos?

Cheering

Seriously though, both make sense, depending on which set of characters you're used to. Or rather, neither version can be called wrong.

The difference is Berlin's population or its administration never referred to it as Берлин (save for a 4-year period maybe), whereas Gdańsk called itself Danzig for a long time. I wouldn't agree that Danzig is the only, or "proper" name but I certainly don't see the problem with using the name as such.

I get your point but the Berlin analogy simply doesn't make nearly as much sense so it's not particularly useful for ridiculing SpacemanSkiff.