Pros and Cons of choosing JDO or JPA for a Grails Application that will run on Google AppEngine
I do see why you would choose JDO. Don't know what is the "default" referred to by other poster. JDO is datastore-agnostic. JPA isn't. JDO allows fetch groups. JPA doesn't. JDO allows datastore identity. JPA doesn't. JDO allows datastore transactions. JPA doesn't. All of this is public record http://db.apache.org/jdo/jdo_v_jpa.html
JDO and JPA don't fill the same problem space. They overlap only for RDBMS. JPA is only designed for RDBMS.JPA is not simpler, as evidenced by the API comparison at http://db.apache.org/jdo/jdo_v_jpa_api.html
--Andy (DataNucleus)