by Dave Sherwin.
I always use the Moodle XML format because it is was developed by the Moodle Community. Therefore if you are moving Questions from Moodle Site to another Moodle Site this works best.
The Aiken format is only for Multiple Choice answers. When you have to create many MC questions this is the fastest way to create questions outside of Moodle and then Import
I have never uses either the GIFT format or XHTML format so cannot say much about those 2 export file types.