I'm sure someone else can chime in with a proper answer because I'm not sure on the actual practical difference between each. There's definitely a difference when the jacket is being worn. If you look through this thread and see for example pictures of Dan wearing his X-3 which has curved sleeves, the curve is quite prominent as opposed to the X-1 which has the straight sleeves. For me personally, it's a matter of preference, I like the look of straight sleeves, the fit of straight sleeves, and curved sleeves always remind me of a bulky biker jacket that should have armor. But to say it wont be curved when you wear it is wrong, the curve is definitely noticeable when being worn.