Personally I prefer to utilize the physics engine whenever possible, so BodyGyro, but both ways should work.
That entirely depends on how you connect the turret to the vehicle, how big forces you use and the ratio of the turret’s mass and the vehicle’s mass.
For example, if you added a BallSocket connected to the vehicle’s roof and the turret’s center of mass, and put a BodyGyro or CFramed a part at the center of mass, then rotating the turret would not affect vehicle’s movement at all (though you would still have to consider the center of mass of the vehicle and the turret as a whole).
That entirely depends on your preference. To tweak the mass and center of mass of individual assemblies you can adjust Density or enable Massless property.
Keep in mind that if you set an entire assembly to Massless, at least one part in it will keep it’s mass.