In my honest opinion, go with synthetic wood decking too. I have come to the conclusion that wood is pure garbage, it warps, twists, curves, and does a million other things that I never want it to. I am seriously into not using wood for construction anymore. Of course the uprights and bracing I probably will, although if I find that there is an alternative to that then it is possible I would use synthetic there too.

But, screws for sure, nails outside are a temporary solution installed in a permenant manner. You cannot pull them easily, and they never really seem to stay put. Screws on the other hand, can be easily removed, and they tend to stay put.