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.
From my experience reading has been emphasized to a greater level than writing in the primary grades. The idea is that if students become fluent and competent readers at the lower grades, then this will in turn become a "gateway" to writing. I believe this teaching philosophy does indeed help to foster the genre of creative writing, but not much else. Students need to develop all forms of writing throughout their elementary years. Written communication of ideas is becoming more and more heavily assessed as well. I have found that writing a basic 5 paragraph essay to be very difficult for my 5th graders. I have to take much time breaking down the process, modelling topic sentences, helping them find strong evidence, and reminding them to really explain their proof. I believe this is a result of the emphasis on reading and oral language development of grades k-2. There should be much more writing expected from the lower grades.