I have been told kettle bells. However in my experience I have found them to be equal because unless you pick them up nothing will happen. Surprisingly it is not enough to simply have them in the house!
I would have thought that because you can't lift kettle bells above your head in the same way, or crunch them across your chest when lying flat that there were less movements. It is more likely that kettle bells are capable of these things, but you have to be stronger. Kettles are probably for those who already have quite good upper body strength, while dumbbells are better for those still build up their arm muscles.