Which salt mix to use is one of the most hotly contested and eternally unresolved issues in the hobby. None of the mixes are all that close to natural sea water and none of the tests performed on a wide range of them prove anything as EVERY single salt assay has come up with different "winners". You will also find people having both success and issues with EVERY brand out there. They are all a little different, all can support aquarium life, you just need to figure out which one works for you and compensate for their deficiencies. In you case Reef Crystals might have a touch more Ca. in it than Instant Ocean, so after using IO check your Ca and Alk levels and if they are off adjust accordingly. FWIW, I have used IO for the past 15 yrs. without problem ( and probably more hobbyists by a long shot have too), yet others think it is the worst salt out there. Certainly, it won't harm your tank.
I will say that I have been testing a new salt that is about to hit the market that I really, really like......but more about that when it is released.