Generally, it's worthwhile reviewing a revision just to see what they did with it. While the author generally considers it better, if the plot doesn't change (i.e. the alien doesn't win the girl), the readers is unlikely to notice any difference. However, it gives you another peak into the mind of the author.
But a good rule to follow is just read the first chapter. If they've revised a story, they'll focus heavily on the first chapter, in order to sell the revision. If you can't see any difference, then there's no point in reading any more as the differences are more form than substance.
Many authors read a point where they prefer revising their older works to make them more consistent, style wise, with their later efforts. They'll often mark them as 'revisions' (i.e. new versions of old stories), but that's mainly to alert readers that the stories have been redone, not to denote that their entirely new stories based on the same premise (technically a complete rewrite).