There are two basic "styles" of Kettlebell training (this excludes some of the patently stupid examples of kettlebell technique and their associated certifications): "Hard Style" or the RKC method and the GS Sport or "Soft Style". Both of these styles have loyal, dedicated and competent athletes following them and both will make you both functional and fit using the Kettlebell. Both have solid methodolgies and training methods that allow the athlete to learn and progress from basic to intermediate and on to advanced techniques. Both are also helmed by dynamic and charismatic athletes that can both "walk the walk and talk the talk".
So, why pick one over the other? It's really going to be based on your personal preference. In my case I prefer the RKC method because of it's emphasis on strength, tension and explosive power. These elements are harmonious with the over thirty five years of martial arts training that I have. The GS or Soft Style emphasizes relaxation, endurance and flow. This style of movement is also harmonious with some of the martial training that I have had in the past thirty five years but doesn't fit into the tactical or combat mindset that I bring to my own training. This is not to infer that the GS method doesn't work; the top tier athletes in this sport are amazing and have endurance to spare. It's my personal preference for explosive power, dynamic tension, and combative strength applications that makes me train (and continue to train) in the RKC method. If you are interested in learning how to use Kettlebells with a more of an endurance or sport emphasis you would be well served finding a good GS Sport coach and learning their method.
NOTE: the folowing video gives a pretty good side-by-side comparison of the RKC and GS methods. Take a look and see what you think!