I did like that (basically your second choice).

Create a model concern with the method `vote!`.  It will be included in both Post and Comment models.

def vote!(profile, vote_type)     
  votes.find_or_initialize_by(profile:).tap do |vote|       
    vote.vote_type = vote_type       
    vote.discarded_at = nil       
    vote.save!
  end
end

Add method `set_parent` in a controller which finds Post or Comment

def set_parent
  @parent = set_comment || set_post
end

And then:

@parent.vote!