EC and pH are two characteristics of the nutrition solution that you feed your cannabis plants with, that need to be properly controlled, measured and if needed - corrected.
In order to know, whether your water with nutrients is in the range of 5,8 - 6,8 enjoyed by cannabis plants, you need a proper device to measure it. Those come in two forms - either the less convenient but cheaper markers - papers or solutions, that change their color according to the solution's pH - or a more advanced electrode based ph meter.
Though requiring more care, the professional ph meter indication is more precise and done faster, with no additional costs associated with every reading. Using a ph meter with an electrode you will be measuring ph more often, thus giving your plants extra care that will be rewarded in your marijuana crops.
With EC meters no such dilemma awaits - since EC cannot be measured without the involvement of current, those come only in forms of battery powered devices with an exchangeable electrode.