Mike,
There is lots of good advice here but my suggestion is to build as strong as possible TO YOUR plan. Whether to use L Girder or open frame, use the build plan best for your layout. REMEMBER, you may have to crawl on the layout at some point in the future. I fell through a layout and it took six agonizing months of rehab and excruciating pain to recover. DO NOT make the mistake of saving a couple of bucks at the expense of strength. PLEASE !!!