Decide between definition, volume, and curl
A narrow brush may be easier to maneuver on short or lower lashes; a larger brush can suit someone looking for a fuller-looking application. Those are practical application differences, not guaranteed results. Read official information about the wand and finish, then consider whether you prefer a single coat or buildable application. Curl-focused formulas may also carry water-resistance claims, which should be verified on the exact product and variant. If you wear contacts or have eye sensitivity concerns, inspect the ingredient and removal information and seek professional advice where appropriate. The most useful comparison is one that includes the brush you will actually be using, not only a before-and-after image.