I'm going to cover a few basics that should help you feel armed to make this decision! Sugar scrubs are gentler on the skin and is the only one you should use on your face. Sugar granules are rounder, and tend to be less jagged than salt granules, so they are best for the face and other sensitive skin that may be irritated by a salt scrub. Sugar also dissolves quickly in water, reducing the timeline for abrasion - which can be a really good thing when you are avoiding skin irritation! Sugar also contains some glycolic acid, so it uses mechanical exfoliation with the grains and a bit of chemical exfoliation as well. You can use finer-grain sugar scrubs two or three times a week. Unrefined cane sugar can be scratchier and less gentle, with coarser grains but this kind of sugar can also have the remineralizing effect of salt with calcium, iron, magnesium, and potassium, which nourishes your skin. You won't want to use unrefined sugar on your face because it will be too rough, but it is much preferred over salt if you have freshly shaved legs or a scratch or abrasion, which salt will aggravate and sting.
Bottom line: exfoliate to keep the largest organ you have,your skin, debris-free, circulating, and nourished. It matters more when you're dealing with your face and lips, so be sure to keep sensitive skin unmarred by using small-grain sugar scrubs.