I find anything over about 30" deep is a waste of space. I'd rather have the floor space (for perhaps a scope cart). Other than that, I too prefer the 'L' or 'U' layout.
Plywood is too thin for benches. Solid core doors are pretty cheap and make very solid bench tops. If you're a little richer, Ikea sells Birch, Oak, and Beech solid butcher block slabs pretty cheap, too. I just bought a small one to replace the extension table on my cabinet saw. I'll replace all my door table tops over the next year or so.