Do both the Ground Instructor rating tests....might as well do so since the questions are the same almost as for the CFI and FOI. If you ever lose your medical for some reasons or a job description calls for you having them....you are a step ahead.
You can walk in cold to the Ground Instructor tests after having done the CFI and FOI and pass no problem due to the commonality of the material and questions.
I did that years ago...and still have not used them but it sure makes for page filler material when you start copying licenses for applications.