You can read this article. It provides a very good comparison between Objectify and JDO
http://borglin.net/gwt-project/?page_id=491
It is a late reply but i diged into this issue as you. As i wanted to use lucene and compass at that time for full text searching and then i found you can use objectify in full text searching in this project
http://code.google.com/p/fulltext-search-in-objectify/
