Those are good questions of Mahlon and one can come up with different answers for them, since the strings are so complex. I agree on that with JWL, and also what I am noticing is a really huge, wide-ranging string sound is being formed by all these "sub-libraries" put together (Solo Strings, Chamber Strings, Orchestral Strings and Appassionata Strings) and they each have their different strengths. It so far seems to me that besides small, soloistic things (such as a string quartet or an actual chamber piece) that layering the Chamber Strings or Solo strings with either Orchestral or Appassionata works best in quieter music. Because you cannot crank Chamber or Solo up in volume as high without being unnatural. So if you are doing loud, heavier things, especially a "Hollywood" style film sound, the Appassionata are essential. Though they have their quieter side as well. The Orchestral Strings form a backbone for all of them, almost like a painting medium to mix the other colors in.
So anyway, a good approach could be to determine the scale of most of the music you are writing and go from there. If you are exploiting absolutely every technique, then you want the Orchestral Strings. If you are using more "normal" string writing that is big scale and but also molto espressivo, then maybe Appassionata combined with Solo would be the best combo of any two libraries put together. Though you will be missing some specialized articulations. Of course, you cannot go wrong getting ANY of them. [[;)]]