Are you referring to Reef Crystals? That comes in an orange bag.
Instant Ocean comes in a purple bag. Both are made by Aquarium Systems.
The former has more trace elements for reefs, the latter is more for fish-only, but either will work, you will just need to dose more with Instant Ocean to keep your calcium and other trace elements in line.
The cost difference between salts will probably be offset by the amount of dosing you will or won't have to do - in other words you'll spend the $5 on supplements if you go with the cheaper salt.
Of course you should test before dosing (ie Calcium, Magnesium, etc.)...
Jenn