TDS or EC really doesn't matter as it is a relative measurement that you will learn to use to determine how much nutrient you are using. I don't think the accuracy difference is enough to be that important because even in young plants up to 50ppm inaccuracy isn't going to have a negative affect on the plants.
Both numbers are just reference points so that you can say that the last time you watered with nute solution, you had a certain level of nutrients within the solution. Different plant strains will have different levels of nute need and there are many variable that will affect a given plant's desire to eat what you put in the soil. It is most important to keep a running journal of what you do so that you can go back to it later if there is a problem or question of what you did in the past with a certain strain.