Metal are far superior to plastic and less likely to unravel when opening or closing. Too, they tend to be more consistent when it comes to the size when you open them up. However, if you are planning a cane manipulation routine or wish to do a color change with a cane, the plastic is best due to weight and ease in set-up in scenarios like the color change.
The plastic Dancing Cane is a pain in the rear in that they don't balance easily and you practically have to relearn your control every time you open it up.
The four biggest down sides on the Metal Cane is that you must be religious when it comes to keeping it oiled or lubricated, especially if you live in an area with heavy humidity and heavy rains or mist. This of course leads to the #2 problem; not leaving them so oily as to stain any silk you may use with them and/or cause problems with any livestock that may be used in conjunction with them. #3 is the fact that the metal cane can and probably will cut you eventually with 4th place going to the classic instance of the thing flying out of your hand due to improper placement and hitting someone else, causing injury. . . there's actually a famed story about a magician coming home from work one night that was approached by a mugger whom he over-powered by using the very tactic, shooting the cane straight into the mugger's eye.
I hope this answers your question.