Starting with Stage 3, all AAC applications are for a sequenced Unity program. There are no word endings to learn in any of the Unity 1-Hit programs. Some people may maintain their language development level at Unity 1-Hit. Many need to continue on to a sequenced Unity program. If possible, start with a sequenced Unity program at Stage 1. With this approach, there is no transition or re-learning to do.
For language stage 3 and above, use any Unity Sequenced version. Choosing the program with a large number of keys will provide a larger vocabulary than programs with fewer keys.