Generally reef salts are supposed to have higher levels of calcium, magnesium and other trace elements that coral needs to thrive. I prefer Tropic Marin Pro Reef but have been thinking of making the switch to oceanic because of how much cheaper it is.
at my LFS there is $3 difference between the two and I do with the Reef Crystals. The fish, inverts and the corals that I have seem to do much better with the Reef Cystals.