the Zip is your basic pedometer. It doesn't really have any bells or whistles. Clip it on your hip and go.
The One (which is what i have) can count steps, stairs, sleep, and calories. It also has a pretty little flower in which I earn petals. It is not designed to get wet so you can't swim with it.
The Flex is a bracelet. I personally chose against it as I like to wear jewelry and this may clash. It is water resistant but it doesn't count stairs.
it is a personal choice. What do you want it to do and will you really use it. My little buddy and I are inseparable except for the time I take a shower or have to charge my buddy up.