Hi, L. The choice of wood, metal or a combination is strictly an architectural decision. Your choice will be dictated a lot by your style of house and decor.
Building stairs, and spiral stairs in particular, requires specialized equipment in most cases. There are ads for spiral stair manufacturers in home building and home improvement magazines and, I'm sure, on the web as well. There are many metal or metal and wood combinations available in kit form and these are relatively economical. If you are looking for stairs built to your design out of local or specialty woods, they are available for big bucks, usually. Your best bet is to do your homework. A good place to start for local sources would be the upscale independent lumber yards, the ones that yours upscale builders use. But in most areas of the country, the good stair builders are usually many miles away.