Currently it seems to default to whatever I used a week ago.
If I set the dual units to mm then close the measure box and then measure something else it's not set to mm, it's set to whatever it was 3-4 changes ago. It does eventually usually change to the last used but why doesn't it save the last selected units?
I was just doing some architectural modeling for out new building and every time I closed and reopened the measure dialog box the dual units option was set to mm. Every single time I had to change it to "Foot - Architectural". Like 5 times in a row. It's really annoying.