EWI does not make their cases, they're OEM manufactured by Road Ready. EWI's prices are definitely lower though.
Also, they're not 3/8" ply. They're 5/16" ply with 1/16" laminate. The total thickness is 3/8".
I'm not knocking anything though, EWI/RR cases are definitely the best buy, but only for a specific group, like muso's and regional/local sound companies that don't work with Union crews & loaders. Unless they're housing something lightweight like a couple wireless units or some rock stars wig collection, 1/4" ply cases and single-wall extrusion cases usually won't last one abusive tour. Back in '93 I had to have my lower leg X-rayed when one came apart and landed on my shin. EWI/RR cases have a slight extra thickness, and have double-wall extrusion, so I believe they'd probably last a bit longer.