The useful choice here is fit. Regular gives more room through the torso, while Slim trims it down without changing the shirt’s purpose.
Poncho describes their classic western shirts as using the same lightweight, breathable, quick-drying fabric found across their lineup. They also list built-in sun protection and functional details intended to stay comfortable when you’re working up a sweat. Pearl snaps and western styling set it apart from a standard button-front if you prefer a more traditional look over a technical one.