Wood has allot of downfalls for siding. It doesn't hold paint very well. It will rot in time and the bugs love it. Plywood is easy to install and adds tons of strength to your building. It's relatively cheap and you can get it anywhere.
Planking is very expensive and has all the other problems with wood.
Metal is a good choice for durablity, but appearance is up the you. Either you like it or your don't. I don't care for the way it looks on sideing, but thats just me.
I've decided to use James Hardie HardiPanels. They are cement fiber sheets 4ft by 8ft and hang like plywood. Unlike wood, they are waterproof and require no vapor barier behind them. Just nail them up right to your framing.
Price is a bit more. The wood grain sheets are $22 each and the ones I'm buying, stucco, are $25 each.
Here in the south, humidity and moisture are an issue, anything that comes with a fifty year warranty is exactly what I want.
Eddie