The barndoor and reflector are fixed to help us shape the scattered light and direct light to some particular places we want to highlight. Certainly, the barndoor is more flexible because of its physical design. We can easily change the range and angle of the beam of light to create a specific ambiance or special effects, adjusting barndoor alone, or with other auxiliary devices.
The reflector has more efficiency due to its parabolic structure and material. Making a comparison with the barndoor, a reflector can provide a stronger beam of light which is more directional. Thus, if photographers can take advantage of reflector's features well; it's not hard to achieve a desirable environment.
Obviously, barndoor and reflector has their own trade-off to obtain individual effect. Deciding depends on environmentally external factors and what kind of atmospher want to be made. Therefore, entirely understanding their features is the first lesson. After that, photographers can mix them up to create various, unique styles that belongs to themselves only.