Another thing you should think about is that the decking size planks of most of the synthetic material is what we call "5 quarter" and is clost to 1" thick. You need access to a thickness planer as well as a table saw (or router) to machine the rabbets in the sides. Lastly the width sizes in the stuff I've seen is limited. It's worth thinking about because of the movement associated in using wood in an outside environment is a real pain.

John Atherton
Wichita Forest Woodworks

I know you believe you understand what you think I said, but I'm not sure that you realize that what you heard is not what I meant!