Different salts will mix up high or low on different things that are very important for reef chemistry.
For example, Instant Ocean generally mixes up low on calcium but with relatively high alkalinity. Oceanic mixes up exactly the opposite. There are big differences in various trace elements, but nobody is really sure how a lot of those elements interact with reef animals as long as they are below a certain concentration or in a certain range.
If you're really serious about your salt, there's plenty of info out there. I personally think most reputable salts are going to be ok to use. Instant Ocean, for the money, is hard to beat.
That said, I just started mixing Instant Ocean and Oceanic at 1:1 for a couple of reasons: 1) their mixtures of calcium and alk level them off to just about natural sea water, 2) they are readily available locally with IO available at Ebay and O from the Critter, 3) if I ever get a bad batch of salt, it will have about half of the effect that it would if I were just using one salt brand.