Having taken a look at the various options of standing seam sheeting, I'm not happy with how they recommend to install it. There is no air gap between the breathable membrane and the steel sheeting. That sarking board/osb/ply will be mush in 15 years. The breathable membrane won't be getting any favours with the heat transfer either. I can see why they recommend to install it that way, due to the need for support underneath.
I'll go down the corrugated route I think, as that will give the most strength and should last a good length of time with minimal maintenance.