Based on your comments, perhaps strongly consider the Special Edition extended library. Especially as it sounds like you could use a wide spectrum of instruments for emotive composing, the Special Edition extended could be ideal. It will not only give you chamber strings, but also basic articulations for most of the secondary "color" or emotive instruments.
The two SE libraries could serve as your foundation, and then you could add full packages as budget permits, and as you determine specific needs. If you choose AP Strings extended, it would help with additional string techniques. The Special Edition extended library would likely be of use in a greater number of situations.
Also, if your computer resources are fairly modest, the SE libraries require considerably less computer power than the full libraries.
Ultimately, if possible, it is generally suggested that for completeness, within the full libraries, both the standard and the extended sets are needed for ultimate depth as well as for maximum articulation options.