I already have a basic OTO/PH test kit at home so I am trying to decide if I need the TF-100 or the TF-50? The only differences that I caught for sure is the TF-100 includes the basic test kit I already have as well as more solutions to complete more tests...but since these things expire over time do I really need that many extra? Is it worth the extra $23 for the TF-100?