Read the fictional version first.  Fiction is usually easier than nonfiction.  That will build background knowledge to build from when your child is then reading the nonfiction book, as the story will only give a few facts and build a context for those facts.  The non-fiction book might be so much information that it could be overwhelming.  Finally, the answer to your question would be different depending how much he/she already knows about the subject.  If your child knows a lot about dinosaurs, the book's level is decreased.  Book leveling is not accurate.  It all depends on the information the child already is familiar with.