For my use, I chose plywood on an open grid frame. I knew that I would eventually be leaning on it and I even have some areas that I had to get on top of to do the initial trackwork and scenery. Another consideration that I wanted to include was the ease of mounting turnout motors, Turntables, and other accessories. Most importantly...I just wanted something that would standup to the bumps and abuse that I may give it.
I live in the heart of the South with very high humidity and haven't experienced any problems with the use of plywood. The most important thing to remember with plywood is: buy a good quality BC rated, 1/2" thick min., support it well with a good frame 16" centers max., use plenty of screws to hold it down.
I don't like the mess of working with foam, but I have seen some beautiful layouts that use it. A lot of guys like Homasote and I may have used it if I had been able to find it in my area.