Weapon family numbers do not indicate level of advancement any more than component family numbers do. They are just integer labels. The AI uses weapons of the family that match the family called for in the design creation file.
Majority Weapon Family Pick 1 := 9
Majority Weapon Family Pick 2 := 1
Majority Weapon Family Pick 3 := 25
Majority Weapon Family Pick 4 := 0
Majority Weapon Family Pick 5 := 0
The lower numbered pick is given priority; if a weapon of weapon family 9 is available, none of the other types will even be considered. Once the group of possible weapons to use has been determined, the metric for picking the most advanced is the same as for valueless abilities as as outlined in the following FAQ apply to weapon families and determining what to use:
http://www.spaceempires.net/home/ftopict-448.html