Do you find that you need elevated Alkalinity, Calcium and Magnesium levels? Generally, that's the reason for switching. If you're happy with the levels IO is providing there really isn't a reason to switch to Reef Crystals.
As for other brands it really all comes down to the levels you want out of your mix and personal preference. Some salts are sold as a higher "grade" than IO products meaning they are a little more stringent on ingredients which can be beneficial for some.
Personally, I chose my mix simply because I have a large mixing container and am lazy. In being lazy, I don't want to clean my container, so I chose a mix that keeps it clean :)