Janet Rosen

Hi, Sue.
For me, one big difference between styles is pedagogical: how are newbies taught? Yoshinkan has a very specific syllabus of techniques through which one progresses, learning proper hand and foot placement and movement. Ki Society has ki exercises that are a specific way in which students get a handle on incorporating breathing, relaxing, centering into the techniques from the get go. Under the Aikikai umbrella, things are a lot more variable: one group, for instance, might stress proper step by step form of basics like suwariwaza shomenuchi ikkyo; another might more explicitly integrate relaxation and flow right from the start.
Each of these will appeal to some folks' learning styles and/or goals, and not to others. It's one reason, if there are options for training in a person's area, the best advice is to visit and watch a lot.