it depends on what weapon is fired, at least in DA and TA.
On-Type Defense is almost always better than having Off-Type, but having Off-Type isn't completely useless- as its sq. root value is added to the total defense of the On-Type.
Shields offer full defense to Beam Weapons, and Sq. Rt of defense to Mass Drivers and Missiles.
Armor to Mass Drivers
and Point Defense to Missiles
Its always "best" to build a ship that counters whatever weapon the AI is using against you. Very seldom (almost never) will the AI use multiple weapon types against you, making it easy for you to build ships with that specific counter. Now admittedly, the chance of multiple AIs using the same weapons is much lower. (So you may jhave to make ships with Shields against the Drengin, and ships with Armor against the Yor)