I prefer open grid, but on a previous layout did use L-girder for a long-span area which included a duck-under.
For my current around-the-oddly-shaped-room layout, open grid is more suitable,as it's easily tied to the wall studs, so that even with a generous overhang of the layout into the aisle, the grid remains sturdy enough to lean on the overhanging portion, or stand on the fully-supported areas (now mostly covered with track and structures, leaving little foot-room).
L-girder on this layout would be simply a waste of good-quality Select Pine, as I chose to support the layout mostly with 2"x4" kiln-dried lumber mysteriously left-over after I had built my house. In addition to supporting the layout, it provides a good platform for under-layout storage...
The open grid itself is Select 1"x4" Pine. I also used open grid for the partial second level of the layout. Done in 8'-10' segments, the front (aisle-side) and end members are 1"x4", while the rear and intermediate members are 1"x2", the latter screwed to the wall studs and the entire grid sections supported by welded-together 1.5" angle iron brackets, lag-bolted to the wall studs.
I can't imagine anywhere I'd need to use a long span, so I wouldn't waste money creating L-girders when I could simply add a few more legs, made from cheaper dimensional lumber.
As I understand it, L-girder's main benefit was the ability to easily move crossmembers if turnouts or other similar things happened to fall directly over them, making under-layout switch machines, etc., difficult to install. You simply unscrewed, from below the layout, the crossmembers which were in the way, then re-aligned them as needed, re-installing the screws....with open grid, it's only slightly more work, although I have only one under-layout switch machine, and its location was well-thought-out in advance.
Wayne
Our club layout peninsula which is roughly eight feet wide x 16' long has four 16' L girders made from 1x4s each supported by just two 2x4 legs. It is as solid as a rock. We could have gotten away with just two L girders but we wanted to be able to split the peninsula in half if we ever have to move the layout.
The rest of the layout has simple 1x4 gridwork. It is as solid as a rock too.
You choose.
I would suggest that the benchwork, regardless of the design, be attached to the walls in a few places just to prevent it from moving around.
Dave
I'm just a dude with a bad back having a lot of fun with model trains, and finally building a layout!
Scroll down in this forum, just a few threads down is a thread titled: "How long for an unsupported span of L girders"
It addresses your questions.
Mike
My You Tube
Personally and respectfully, I prefer I beams. They're engineered and can be modified to suit your needs.
There's another thread here that covers this.... L girders are good. I beams are better.
TF
I have always used good old fashioned grid bench work. When I build my new layout, I was considering L girder benchwork. It would not be attached to the wall, but would butt against it. I would use 1X4 for legs.
Caldreamer