Cinematic Strings 2 (CS2) is a great library and has been around for about four years. It has been very popular with students and professional composers. It was recorded in a large concert hall and has a very big lush sound. It is probably slightly easier to use. It is around 22.5GB installed.
Cinematic Studio Strings (CSS) is the new library, recorded in a Hollywood-style scoring stage (where they record film scores). It has the same kind of GUI as CS2 but has more articulations (the list is on the website). The legato engine is a bit more complex but results in amazing realism. It is the first in a full-orchestra series which will all blend perfectly together. We have released the piano library at the same time and Solo Strings will be next (in a few months), and then Brass after that. CSS is about 34.5GB installed.
So CS2 has a concert hall sound, is a little bit easier, a bit smaller, and cheaper. CSS has a scoring-stage sound, has more articulations and more realistic legato. It has a bit of a learning curve, is a bit more expensive, and a bit bigger. If you only want to have a strings library, then either one is fine. If you would like to buy brass and woodwinds etc in the future, then I'd recommend CSS.