dry mixes are cheaper and easier to dose if you use Chuck's calculator. Knowing that 1tsp will give you a 10ppm increase is helpful. With SeaChem stuff it's a little weak, and there's no handy chart to tell you that 1capful is 10ppm in a 20gal tank...they only have guidelines.
so for small tanks, you can get by with Seachem, or kent..but on anything over 20gal, you should use dry ferts.
Good example, it takes about a half bottle (2 full oz) of Seachem's potassium to give a 10ppm raise in a 75gallon tank. that's very pricey at $7 a bottle, compared to $1 for a pound of K2SO4, where you only need 1tsp to give 12ppm of potassium in the same tank.