Except that writing code using JDO/JPA is a nightmare, at least on Datastore. So that's almost certainly not the smart way. I'd rather rewrite my app three time

Except that writing code using JDO/JPA is a nightmare, at least on Datastore. So that's almost certainly not the smart way. I'd rather rewrite my app three times - go with Objectify (simple, easy, maps well to Datastore concepts) and just deal with the rewrite if it happens.