
a film score without a budget to hire a real orchestra), I'd probably go for Spitfire.
My very subjective analysis: for scores in which my final product will be the sampled recording (e.g. I know this isn't helpful in making your decision but hopefully it will alleviate some anxiety about making the "wrong" choice
For what it's worth, most reviews/forum answers I've seen tend to conclude "they're both great, just different, so buy both!" and that they layer well together. One YouTube review I saw seemed to demonstrate that CSS can play quiet, tender music in a slightly more, well, tender way-but I don't know if this is a general strength or just the merits of that particular sequence. CSS sounds pretty good with very little tweaking necessary, but you can't get as much variety of articulation or as fine-grained control. Spitfire has more detailed control over the various nuances of performance and has more articulations, which can be very useful or very tedious, depending on what your needs are. They really were meticulous about programming it so you can copy lines from one part to another without having to re-record all of your controllers. CSS is very consistent from patch to patch, which makes workflow incredibly easy.
Spitfire sounds more "realistic" - there's more of a live sound, with a bit more bow and string to the sound that makes it sound more like a real section in a hall, and a bit less like a sampled mass-of-strings patch.(I do have Spitfire Symphonic Strings and Cinematic Studio Brass, as well as the old Cinematic Strings 2, so I have experience with related libraries but not the exact ones you're looking at): My impressions, after doing a lot of listening to demos and reviews.