CFII written
I took it twenty years ago (!), but I doubt things have changed much. In other words, the instrument written, CFI-I written, and IGI written are taken from the same general question bank. Therefore, if you prepare thoroughly and properly for one, you should do fine on all.
Accordingly, why don't you plan on taking the IGI written as well? You just need to review a little and you will be on your way to getting a Ground Instructor certificate with little effort expended.
Good luck on the written(s).