Before describing the differences, let's first state what they both are. They are unique alternatives to traditional papers. They are both machine-made of plastic and are waterproof, stain resistant, and extremely strong and durable. 15pt synthetic paper is made of polypropylene. 18pt and 20pt rigid vinyl is made of Polyvinyl chloride (PVC). Polyvinyl chloride is the world's third-most widely produced synthetic plastic polymer, after polyethylene and polypropylene. The synthetic plastic is thinner at 15pt versus 18pt for the rigid vinyl, and it is also a softer more pliable (bendable) paper. The best way to describe paper is to not do so in words, but to see it for yourself! Luckily, synthetic plastics are rigid vinyl are included in our free sample pack for you to see what suits your needs best. We include 8mil synthetic plastic. 15pt synthetic plastic cardstock. 18pt rigid PVC vinyl. 20pt white plastic. 20pt frosted plastic. 20pt clear plastic.