Yes, right, difference is in ion disassociation. Recall that casein, because of k-casein has a charge, and that other proteins do as well. Those proteins act as receptor sites, sort of reservoirs for hydrogen ions. Another way to think about it is they buffer acid. Other minerals/salts also buffer acids. So while pH measures everything that is already in solution it does it in terms of the water, that is, hydrogen (H+) and hydroxyl (OH-) ions. Whereas TA measures that, plus the solids. In a way you can say that TA is somewhat predictive/proactive because it incorporates the idea of buffers, whereas pH is just a measurement at a given point in time.
I would say either work well when used in the hands of an experienced cheesemaker. My suggestion is to pick one and stick with it.
