Amateur, quick comparison video of the features (and costs) for all three apps
Important factors when choosing an AAC app - Customizability, robust vocabulary, easily accessible keyboard, user's learning/processing style, any visual or physical needs, and ability to grow with the user.
Note - in one comment I mention that TD Snap is not as customizable as the other two, recently I have seen some very custom versions of TD Snap. There may be a lot of work involved, but there are more possibilities than I noted.
