Personally, it depends on the application. If you're looking for space under the tank (ie. a big sump or stacking tanks), then metal is definitely the way to go. Having a 6' span without a center support is the way to go when stacking tanks. If you're looking for a piece of furniture, you could do either. A metal stand can just as easily be faced with nicely finished wood as a wood stand can, and if you know how to use magnets you can actually make the whole exterior removable (I've seen it done and looks fantastic, not to mention how easy the access to the sump is without having to deal with doors). Most people in NA use wood because it's very easy to work with, tools are much cheaper, and it's readily available.