A Difficult Choice
The truth is that all salt mixes are lacking to some degree or another. While some salt mixes may be better than others, no one salt mix is going to perfectly recreate ocean water. So how do you chose a salt mix? You can go about it a number of different ways. You can go on the recommendations of other reefers or of your LFS. If you can find a salt mix that lists its contents, that might be helpful. It's always helpful to be familiar with the manufacturers and their reputations. Some reef aquarists simply choose whatever salt happens to be on sale or is the cheapest and/or readily available. Unfortunately, even if you find a salt you think you like, salt mixes can vary quite a bit from batch to batch (even within the same brand). All you can do is pick a salt that appeals to you (and that seems to work for your tank) while at the same time always testing your mixed water to make sure the parameters for calcium, alkalinity, etc. are acceptable.
Beware of Salt Studies
Dr. Shimek's Study
Over the years, several attempts have been made to "study" or "test" the different salt mixes. Several years ago, Dr. Ron Shimek conducted research to test the different brands of salt with urchin larvae. The study was well conducted and had results with statistical power. However, since it was conducted with only urchin larvae in very small volumes of water, it might not be directly applicable to the average reef tank. However, it's interesting that the "winning" salt of this study happens to be the only brand of salt that doesn't use E.D.T.A. (an artificial water clarifier suspected to be potentially toxic to marine larvae).
The Borneman-Lowe Salt Study
Though the hearts of these researchers may have been in the right place, this study is, quite frankly, worthless. Here is a good explanation as to why: Critique of the Borneman-Lowe Salt Study
Conclusion: In the end, it probably doesn't matter a whole lot which salt you choose. No matter which salt mix you do choose, it's most important that you diligently test and maintain your water parameters. (Be sure to test the tank water and the newly mixed water parameters BEFORE doing a water change.)