I have both a few Semi-Traditionals and one Casual from USA Kilts in my collection. As far as versatility, I definitely recommend the Semi-Traditional. Having the complete appearance of a more traditional kilt, as others have so eloquently described, it makes it effortless to both dress up and dress down. The Casual isn't quite as versatile, at least to my eye, due to lacking the strap-and-buckle closure system (instead making use of hook-and-loop, or Velcro if you prefer) and the aesthetic it provides. I do like the convenience of how easy the Casual is to just toss on and go, and it's a very good option for more physical activities (also due to the different closure system). On the other hand, though, there have been times I've found myself thinking "gee, I wish my Isle of Skye were a Semi-Trad instead of Casual".