I have use L girders and have found them to be a very flexible option if you are building with cookie cotter trackbed.
They are also very strong, I can sit or stand on my L girders which are only supported by 4 legs along a 16 foot length, so you don't have to worry about leaning on the girders while working on the trackbed and layout.
Cross members and risers are also easy to move and adjust if you have to for levelling, or changes.