It matters until 80/128, ie 62.5% of range. Smash DI will give you the same distance per input past that number. Slide DI is conceptually the same as Smash DI, except you change directions to allow more inputs. It's like you smash DI, but since you change directions between every input, all of them are registered and you don't have to wait an empty frame in between (as you would with regular smash DI). Imagine you have a 10 frames hitstun move. Smash DI implies you have to alternate between empty frames and input frames, so you can have 5 DI input. Slide DI allows you to move every input, so you have 10. However, you can't say slide DI is exactly twice as good as smash DI most of the time. Imagine you want to slide DI left. You have to alternate between up left and down left. Because of the shape of the joystick turning (circle and not square), when you're up left or down left, the projection on the x-axis of your 80/128 of range makes it only 56.57 (cos(45)*80). So if you have a borderline range, even though smash DI could give you the maximal direction per input, slide DI wouldn't. So even if you have twice the number of inputs, it might not go as far.
Everything put in the balance, I still don't see a reason to smash DI over slide DI, especially since the range is rarely that low (or you wouldn't use that controller anyway).
