This is a difficult question to answer, because the answer will be rooted in the final analysis on the nature of the student and the culture of the school. However, if I had to take a stand, I would say that reading is preferred by most elementary student for three reasons.
First, children learn to read at an ealier age than they learn to write. So, by the time they are in elementary school, they have had much more practice in reading than writing. And as a general rule of thumb children (adults included) like to do things that they are relatively good at.
Second, there are so many good books out there to grasp children's attention. Think of books like the Hunger Games, Diary of a Whimpy Kid, and the like.
Third, writing is a more advanced and difficult discipline than reading. For this reason, generally speaking children like easier things. Keep in mind that writing frustrates even adults many times.