Well, it depends on what you want.
The basic 250Hz regular version will give as good a workout to your arms and wrists as any of the others (except the metal). It's also fine for rehabilitation exercises, and will probably suit people with smaller hands, as there is no counter on the top.
It you want to know how fast you can spin your powerball, then you will need a Pro version.
Fancy some flashing lights as well, the Neon version is what you need.
Want to make some noise, go for the Screamer.
Already got arms like Garth? Perhaps the awesome 350Hz Metal is for you.
For out and out speed, the 250Hz Pro is rated as the fastest, as the Neons take some of the rotational energy out to power the LEDs.
Still not sure? Give us a call, we are happy to assist.