When I posted the questions in the thread that Bob referred to I was initially keen on replacing one part boards with two part ones but I went with the advice of one part boards. That was on my mkII. I now have a mk1 which has two part boards. I have found that the aft ones flex a little more than the one part boards despite having battens underneath to stiffen them up (I sail with the aft benches removed so perhaps there is a tendency to tread a little closer to the edges of the boards). I dont know what grade of ply they had been made from.
So, having some experience of both versions, I too would probably conclude that one part boards are the way to go.