16kg is fine. Of course it all depends on what movements you're doing with it and how much strength do you have before you start training with it but 16kg is good. Until it some day is too light but better that than too heavy and not being able to do more than one or two reps. And although you could do some movements with 20kg, more that two, 16kg is still good to have incase of a day when you need to do things lighter. And it's a good thing to have at least three different weight kettlebells anyway. Ones that are not too heavy or too light. Aaaaaaand, when you get bored with doing things with just one kettlebell, you have to have two of each. :D