Consider the size of your dancers. Make sure that what ever you pick will be flattering on all sizes. If a dancer is in between sizes, always go with the larger one. You're better off pinning or tacking it to fit rather than trying to squeeze into something that's a little too tight, it will look better too.
You also need to think about the style of dance being performed and what movements will be done. Turns are very effective with flowing skirts. If there is a lot of floor work, make sure your dancers won't get tangled up in their costume. Pick something that is more fitted to the body, and if you pick a dress, pick one that isn't floor length.
Remember to practice in your costumes to make sure you can do all the moves required and to make sure nothing pops open or comes off, that gives you a chance to safety pin or fasten whatever might need it to keep it from happening your actual performance.
Good luck!