The semi-trad uses a strap-and-buckle fastening system and the casual uses velcro. The semi-trad looks like it may use a bit more fabric, as well.

I have a USAK casual and am very pleased with it. Unless you're a real stickler for detail I doubt you'll notice the lack of straps once you add a belt and sporran.

Also, Rocky will make you a casual with no logo, just ask in your order.