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kotlinlang

I see 2 more often but with the children being data classes. But in general option 2 gives you more flexibility on how the children need to implement it, but is more verbose with the override.

indeed 2 seems easier to write but the verbosity annoys me

naively this seems like a case where only one syntax should been allowed

There are differences, so both do slightly different things.  If the field is nullable, and abstract property in the base makes it so you cannot smart cast after a null check.  That's one reason to prefer option 1 over option 2.