Which is better for combat / defence turret or gimbal weapons?
It depends on the pilot's aiming skills and the ship's agility. On the high and low end of the scales things are simple: good aiming skills + high agility ship = fixed weapons, low aiming skills + low agility ship = turrets. Every combination in between is a matter of trying out what works best for you. For example if you are moderately good at aiming but you have an agile ship (Chieftain) you might still be most effective using fixed weapons. But flying a moderately agile ship (Python) you might be better off with gimballed weapons. And flying a low agility ship (T-9) you are most likely better off using turrets.
Gimballed and turreted weapons are sensitive to chaff which reduces their overall effectiveness quite a lot. NPCs always have chaff if you have gimballed or turreted weapons installed. (How do they know up front?)
Lasers and multicannons need time-on-target to do damage and that limits manoeuvring while firing (most for fixed weapons, least for turrets). Cannons, fragcannons, rail guns and plasma accelerators only need to be aimed correctly at the instant a salvo is fired, freeing you up to manoeuvre while they reload.
This all combines with your personal preferences (which will change as you gain experience), so my final advice is: you have to try it out for yourself using the above as guidelines.
--edit: I forgot another important factor. On some ships the hardpoints for weapons are located at places that make using fixed/gimballed/turreted weapons impossible. For example, the small weapons on a Cobra mk3 are so far apart you can only hit another ship with one fixed weapon (=50% reduction in damage). You have to have at least one gimballed weapon at one of those stations. Similar problems occur on the Courier, Clipper, Corvette, Cutter. Other ships have hardpoints on the underside with the hull blocking 50% of the field of view, making gimballed and turreted weapons much less effective there (unless you fly with nose high attitude like NPCs do, but then you can't see your target). This happens on Federal Drop/Assault/Gunship, Corvette, Anaconda, Orca, Cutter (maybe I forgot a few).