I think it is best to take readings of parameter of each brand after mixing with same salinity and in what proportion with water. Then you can compare which one would you choose for your tank. So far I have used 2 different brands and here are my readings.
Brand KENT CORALIFE
Mag 1350 1200
Cal 520 480
Sal 1.025 1.025
Kent needs 2 mugs salt per 5 gal
water while Coralife only 1 3/4
By comparison I can conclude that Kent is more favorable but uses more salt mix to attain same salinity.
Note: The salt were mixed thoroughly before the preparation.