Storybird
Generally I feel that Storybird gave the students a greater incentive to be more creative in producing story content. Using commercial art in a finished form freed the students from too much focus on the illustrations and tended to elevate the content quality. It was also easier for students to work at home on projects started at school. The only way of publishing the final product was through purchasing a pdf, apart from 'screengrabbing' each page which would be fiddly and time consuming.
Storyjumper
Storyjumper enabled more freedom in creating illustrations, and even gave the option of importing clipart or photos. The process for students working at home requires parental assistance and, although very clearly detailed, may challenge some parents' technical abilities. Importing one's own content would necessitate making it a png and taking out the background for a more professional effect. Free pdfs can be created, the only caveat being that an address url is printed on each page.