I know this is a very old thread and long past the point where responses may be useful!!!
But anyone contemplating above and below layouts should check out the layouts of Jim and Linda Quick. Linda built a beautiful layout under Jim's. I believe it's about halfway between the main layout and the floor.
There is an OGR video on their layouts and articles in the OGR magazine archives.
Of course, in responding to the original post, most people on the OGR forum would think the O gauge belongs on top and the HO below.
Jim