You are gonna get a million different responses on what is the best salt. You can't go wrong on any of the major brands though. I personally use Aqua Vitro Salinity, just because I like the fact they individually test every bucket. As for mixing I use a mag 7 in a 33 gallon brute trash can with RO/DI and a heater. I mix for a minimum of 24 hours and make sure the salinity matches exactly with my display.
I use brightwell neo-marine percision blend. I mix 1/2 cup to 1 gallon of RO/DI water then I use Koralia 250 gph powerhead and my deep blue heater all in my 5 gallon bucket and let mix for 24 hours. have had great results and good growth. Watch out though when mixing the salt will react violently to water at first so watch your eyes and skin. Also I have not had to dose mag one time always stays around 1300 with weekly water changes of 10 gallons.