get both..that way you kno what you're puttin into the soil and that way you kno how clean your soil is with the soil tester....It helps, because you should kno what ph your soil should be for your plants to grow a healthy grow and if your plants aint doing so good then you can check out the soil ph and check the ph level to make sure you haven't dumped acid water into your plants feeding...

Oh and the difference between vegging and flowering..vegging is from when you switch the light cycle into flowering..Flowering is basically budding stage when you cut it at a cycle of 12/12...

Peace