Choosing a rainstick depends first and foremost on how you plan to use it. Rainsticks work by tumbling small bits like pebbles or seeds through their specially-designed innards, which means that the longer the rainstick, the longer its sound will last as it takes more time for the bits to travel all the way to the bottom. So if you need a sustained effect, look at something like the Meinl Bamboo Rainstick or Meinl Extra Large Professional Bamboo Rain Stick. On the other hand, if you're only looking for a brief effect, the Toca Junior Rainstick might be a better suggestion.
Would you prefer a rainmaker that you don't have to tip back and forth to keep going indefinitely? If so, the Meinl Stand Mountable Rainmaker is probably the way to go. This wheel-shaped model can keep on turning as long as you need it to keep "raining," and you won't have to worry about the sound changing with angle like it would in a traditional rainstick. Plus, being stand-mountable, it's easy to add to a drum setup - or just about anywhere else onstage or backstage, for that matter.
Rainsticks are specialty percussion tools with applications for just about any type of performance. You can use them to create atmosphere in the intro or outro of a song, for example. Or, if you're putting together a play and you need some convincing live weather effects, a rainstick can give you that in a pinch without the need to prepare a recorded rain sound in advance. Beyond all that, they're just plain fun, and great additions to classrooms as well. So no matter how it turns out to be in your case, there's always a way to put rainsticks to use!