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The saltwater system uses the salt to generate chlroine which serves as the primary sanitizer. The ozone system and the Nature 2 are both supplemental systems. The Nature 2 doesn't do anything beyond what chlorine alone does, and adds the risk of staining the pool and swimmers' hair and fingernails green.
Ozone does give added benefits, but the extra cost is usually not worth the benefit unless you have a lot of guests in your pool. Ozone can help eliminate contaminants and is more effective than chlorine on a couple of human transmitted organisms, but for most people, the risk of ever having those organisms enter their pools is slim.