Most of the bags of this type are made of hefty nylon, and many are made of synthetic leather. Both materials are tough, but nylon wins out on giving you good wear. But if you are going for good looks... nothing beats leather (like a leather vest). Don't forget to keep in mind that almost all sissy bar bags are only water resistant not waterproof.
Your motorcycle sissy bar bag capacity options increase to the point where you can get a single bag or multi-bag system large enough to carry all your gear for an extended trip.