Call me old-fashioned, but I still build the framework and subroadbed per the L-girder design that’s been around since the 60s. I usually use 3/4" ply for the subroadbed, because I like being able to count on the extra strength it provides. I’m usually not crawling around on top, but do like to be able to lean on things without them moving around.
For the scenery base, I use 2" extruded foam mostly, a little 1" where appropriate. My latest construction tends to make these removable liftouts, so some 80% plus of my Cascade Branch is liftouts. The base is closely fitted, then the terrain is built-up using the usual glue-and-skewer methods. Over the top of that goes Sculptamold, which results in a strong liftout that is easy to scenic and holds trees, etc well.
Lots of advanatages to the result. You can add track and wiring later, easy access for maintenance, take the scenery to the workbench instead of leaning over the layout, etc, etc.
More pics of this are in my Cascade Branch thread:
http://cs.trains.com/mrr/f/88/t/219241.aspx?sort=ASC&pi314=1