While the question of wood vs metal (or even composite) is a good one, the answer is very much dependent on the rider. While wood is light weight, and generally less expensive than metal framed, it is also prone to greater wear and easier to damage. Metal frames have a much higher weight tolerance while the wood frames are better suited for smaller riders. Composite frames are neither weather or rust affected, where a metal frame should be more rust resistant, than rust proof, and in the same vein wood can take damage from excessive wear and weathering.
So they all have there issues, but which one is the best for you? If the rider is a small child, a wooden version is probably a safe bet. Lighter and less expensive it allows for the rider to get his or her “riding legs” without the upfront cost, and extra weight with metal and composites. If your rider is a little heavier but requires something light, a composite bike would be ideal. Lighter material and strong construction make it ideal for mid range riders. For older children up to adults, the metal frames offer weight support up to 100lbs, which is more than enough for most riders.