I went with the 5/4 decking, ripped in half and bull nosed (rounded) the edges.
It has it's good points and it's bad points. If I didn't need them done quickly at the time, I would have gone with fiberglass. If I ever do it again, I will go with fiberglass or hardwood.
Good: It's cheap, plentiful, easy to work with, and looks almost like the original wood, when done right.
Bad: Modern treated lumber has to dry out before it will hold paint, it will twist into a pretzel while you wait for it to dry out, and it eats steel.
You have to use coated, or stainless steel bolts, and you should paint the uprights where the wood makes contact. I didn't, and I don't know what the long term effect will be on the steel uprights, but I know it's not going to be good. I'll probably take it apart and paint the uprights when I paint the slats.
It's also thicker than original, so you have to plane it down to fit into the uprights on the headboard.
I'm going to paint our this winter, I think they've dried out enough.
Edit: On the second pic, you can see where one of the slats to the left of the window opening warped after only about a week.