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Understanding INNER JOIN versus IN subquery

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I have two queries which should be equivalent.  The Planner plans them differently, although they are both about the same time.  Can someone explain why?
select word from dict
where 
 word in 
 (select substr('moon', 0, generate_series(3,length('moon'))))

select * from dict
inner join (select substr('moon', 0, generate_series(3,length('moon')))) as m
on dict.word = m.substr 

Is one preferred?

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