Whats is the difference between cordless vacuums of different voltages?

I am doing a project on cordless vacuums and need some clarification on what the difference is between the stated voltage. I have noticed that at the lowest point they are around 6 volts, then 9.6, whilst others can be up to 18 volts. So what difference does this make to the function of the product? Does it mean that it will have better suction or just a longer lifespan? And at what cost does the difference make (i.e price, weight etc)