Greetings..
The Goal: to acquire the most effective self-defense system..
The paths in question: internal or external..
The evidence: The highest demonstrations of each path..
The variables: Time, amount of time necessary to cultivate skills representative of high levels of either path.. The cost, in money, in harm received during training, in requirements of the teacher(s) that a student adopt their beliefs, the student's agendas.. available resources/teachers..
Both paths have remarkable demonstrations of high-level skills.. there are frauds in both paths, cults.. Typically, the external path is substantially shorter in gaining effective skills, not always, though.. Typically, the internal path is more well-rounded, with medicine, healing and character development, not always, though..
The first step: Sincere evaluation of the student's goals, sincere research and awareness of hype..
Some of my experiences: that external focuses on alignment and muscular control/development.. Internal focuses of cultivation and refinement of the energies that power the muscles and senses.. both focus of technique, both retain techniques that are not particularly useful in today's reality.. There some styles that try, with some success, to balance internal/external..
I think balance is the key..
Be well...
TaiChiBob.. "the teacher that is not also a student is neither"