This is a great question! It's one that can be a little confusing, and that's because it depends on something that's a little different for everyone and each task - how much space do you need between your work and the lens?
The guiding rule that normally applies is that higher the lens power, the closer you'll need to hold the lens to your work or the object. So typically, a 5 diopter lens will work better for crafting, where you need to examine smaller details closely, and a 3 diopter lens will work better for reading, where you can hold the book a little farther away.