I was thinking that they all had to sound different due to being made of different materials.
This is a widely held misconception. If an instrument is made entirely of natural wood, which has, if uncoated or unpolished, an inherently slightly rough surface, some of the higher harmonics will possibly be muted, giving what may be termed a "darker", "more mellow", "smoother" etc etc etc tone, but any surface that is hard and shiny will give very similar results if manufactured in the same manner.
As for the "resonance" of the instrument ... well, maybe on something as long as a clarinet or full-size flute you might notice a difference between thin metal and thicker wood, but for something as small and effectively "solid" as an ocarina, the design of the windway and "window" will be the major deciding factor in the tone produced, irrespective of the material used, assuming the finish of the material is similar, ie "shiny" or "not shiny", for want of a better term.
None of this is intended to discourage the purchase of a high-end Mountain Ocarina, they're fine-looking instruments, but they really ought to be bought because they look nice, not because they're likely to sound better or differently to any other Mountain Ocarina, though, as Harp Player has pointed out whilst I'm typing this, the extra weight may make the Warmstone easier to handle ... a point I am considering !