Lens Recommendation

If you have to do without flash, then staying away from zooms might well be a good idea.

If the web image you posted shows the sort of framing you want, then a 50mm will give that framing at (very) roughly 15 feet and a 85mm will need roughly 25 feet. I'd say to go with a 50mm unless you know that you'll be forced to stay back 20 feet or more, or unless you want to concentrate on head-and-chest shots with few or no full-body shots of the dancers. If you want to take mostly head-and-chest shots at maybe 12-15 feet, and then step back to take some full-body shots at roughly 25 feet then go ahead and get a 85mm.