It depends on what you plan to do with the kettle bells and perhaps where you are located. RKC and WKC are the two most recognized kettle bell certs in North America; however, each of these respective organizations have different philosophy.
RKC is strictly a hard style kettle bell training methodology. It’s established by Pavel Tsatsouline, and its method is ideal for developing a variety of exercise goals (e.g power, strength, weight loss, short term muscular endurance). The shape of the kettle bells used are (not surprisingly) the hard style versions, where the handle and horns are thick and can be a bit wider than the bell. The kettle bell used generally increases in size as the weight increases.
WKC is strictly gyra sport (i.e. kettle bell competition). This organization is established by Valery Fedorenko, and its method is ideal for those looking to compete or establish aerobic endurance or power endurance through the use of kettle bells. The shape of the kettle bells are the same no matter what the weight of the bell is. The upper limit of the kettle bell does stop at 32 kg, since nothing heavier is used in competition (at the moment). If you want to train with heavier kettle bells, you will have to eventually switch back to the hard style kettle bells.
I would say more people choose RKC over WKC. Generally, if you want to develop proper technique for kettle bell, RKC will give you that foundation. WKC is good in its own right, but the technique used is to train people for gyra sport. Most beginners looking at how a WKC person train might mistaken their technique for sloppy form or get the wrong idea of how to swing the kettle bell.
If you are up in Canada, Agatsu Kettlebell is the largest kettlebell certifying body in the country. Agatsu is largely hard style (like RKC), but will introduce gyra sport to their member, and will encourage their members to pursue a WKC if they so desire.