Competition Kettlebells have the exact same size, shape and handle thickness regardless of weight changes. That means that you will not need to change your grip or forearm position, and can focus on consistent technique no matter what the size. They are also constructed from steel, which is longer lasting and more durable than cast iron.
On the other hand, Cast Iron Kettlebells will increase in physical shape and handle thickness as the weight increases. The space between the handle and body vary slightly with each size. While Cast Iron Kettlebells are quite cheaper than Competition Kettlebells, they are still constructed from solid cast iron material and coated with durable powder-coat for a long-lasting finish.