Resin is strong and rustproof. Resin looks great and is now the leading material used in above ground swimming pools. Most above ground pools have resin and/or resin coated parts. Hybrid pools are very common which feature both resin and steel parts to give you the best of both worlds.
Steel has constantly been improved with the development of new protective finishes and coatings. If you plan to purchase a salt water system, I highly recommend you choose a pool made of resin, since, resin is rustproof.
