Can someone explain the difference between these 2? I think I've done the fx3.1 to my F90bt a couple years ago and wanted to update my f900 but I'm not sure which one to do.

The F700, F900, X710, X910, F90 share the same system that being said the 3.1 or the 3.0 will work with yours. i'd dont remember whats the difference but just use the 3.1.