The beam turret seems to be the most useless to me. It'd be perfect vs. enemy capital ships, but hitting three times on an enemy fighter? Good luck. That thing will be long gone if it's being flown by a human pilot.
the "machine gun" turret seems to do reasonably well vs. fighters, but the damage is again pretty low.
Mostly, the turrets seem pretty pointless and I hardly ever see bombers attacking cap ships anyway. You're better off using a few interceptors around the capital ship to take out bombers, which seems to be what everyone does anyway.
>sigh< Another disappointing aspect of space combat in this game.