The experience of each physician instead of a global generalization of one type of doctor being the “better” choice than the other would better answer this question.
Understanding the true anatomy, and more importantly the variability in the anatomy between patients – for brows, the forehead and anywhere else you plan on receiving a Botox injection is the key. This would include having a firm understanding of the origin, insertion, and action of each muscle that will be injected, the thickness of each muscle targeted, and the patient variability therein.
As a plastic surgeon, I’m intrinsically biased since I operate in the area for browlifts and facelifts and have a unique perspective to the muscle anatomy. I commonly dissect under the skin and see the actual muscles themselves. For me, this helps guide where to inject and where not to.
With that said, however, I know many non-plastic surgeons who know the anatomy well despite not operating in that area and get great results.
The experience and education of the individual physician you seek will guide your decision-making more than their specific field of training.