Martial arts (excluding combat sports) are not, nor should they be considered, anything other than an opportunity to increase an individual's chances of survival. If a student trains under a quality instructor for a period of time, then, at the end of that period, provided the student is capable of learning, his chances of surviving an encounter that requires the use of force should be better to a degree relative to the period of his study.
The art, in this case, should be considered a complete art, and offer a well-rounded curriculum to create a well-rounded martial artist. If the art, for instance, only taught tackling, then the martial artist may become extremely adept at tackling, but extremely weak in avoiding blunt force trauma to the cervical spine. This is, of course, an extreme example, as no art teaches only one thing (that I'm aware of, though my inbox is commonly flooded with emails to teach me the 1 defense technique I need to know to survive any encounter).
The instructor is key, whereas the art is not. A complete art is made incomplete by the instructors lack of knowledge in one area. Further, the attendance and capability of the pupil is a heavily deciding factor. If the class is moving through a manual of techniques in order with each class, and he covers the curriculum from mukyu (no rank) to ikkyu (1st grade before black) (NB: These ranking systems are used to exemplify beginners classes, wherein more advanced beginners [low dan ranks] would not be learning new material), then he may have had an opportunity to train each technique only a few times in those six months, thus effectively having too little time to develop muscle memory in any one movement. On the other hand, he may train in only one technique for those six months, which effectively makes his training no different than that of the proposed 1 technique art above.
Every art has its strengths and weaknesses. Over the centuries, older arts have filled in the gaps to make them as effective as possible, while newer arts, via one means or another, find themselves functionally similar to these older arts. So long as the art is taught well, the chances of success in self-defense will be greater than the student had prior to his training.