Rain water is much more likely to be contaminated anywhere that automobiles are being driven. The rain falling over the ocean doesn't have that issue.
It isn't like comparing apples to oranges. More like comparing organic peaches to industrialized peaches. One has more of man's chemical fingerprint and the other has less. If you decide to risk it, carbon filtration would still be important to use before mixing it with salt. Rainwater always collects dust at the very least.