Multiple individual kettle bells. The reason is that if the weights and handle are adjustable, then the kettle bell will have a different shape with each weight or handle change. You would have to alter the way you handle the kettle bell each time you change the weight and/or handle. Not exactly the best use of time if you had to retrain yourself to accomodate for the tool.
Remember that a kettle bell is not a dumbbell or a barbell. Both of those equipments are designed for symmetry and balance. In other words, your form should not change just because the weight gets heavier. The way you move a 1 lb dumbbell is the same as a 100 lb dumbbell.
Not so for kettlebells. A 2 kg kettle bell won’t move the same way as a 16 kg kettle bell (especially if one is using hard style kettle bells, in which the size increases as the weight goes up). If you are investing in kettle bells to use, definitely use the pro grade kettle bells, where the size and shape remains the same regardless of weight. You’ll spend less time having to relearn how to use a kettle bell each time you move up in weight. It may cost more, but it will be better for your training (and that of your clients) in the long run.