Which metal snaps you use depend mostly on your personal preference and on which size best complements your project.
Open-ring grip snaps are commonly used on baby/children's clothing and other basic crafting projects. Not suited for thick material. Size 14 (9mm diameter) and size 16 (10mm) can generally be used in the same applications.
Utility spring snaps on projects that call for a more heavy-duty snap, or with thicker fabric like leather. Size 20 (12mm diameter) is more suited for general use while size 24 (15mm) is recommended for heavy-duty applications.