I, too have all three.
I just finished an article for Electronic Musician magazine that covers the "best of..." in string libraries that, of course, covers all three of the above-mentioned libraries as well as others.
I agree with KI; SI sounds great out of the box, but has no tool for overcoming the "wah-wah' effect of the static attack envelopes. GOS and VSL are employing revolutionary techniques that no one else is doing. These techniques make the control of articulations much more a function of the user's skills, patience and time.
VSL still has a lot of holes to fill in the areas od dynamic layers, articulations, etc., which they are quite aware of and are working on like crazy, I presume. The promise of where VSL is heading based on the initial release of the Cube and P-Set tells me that VSL will need to be a component of all serious users sooner or later.
That being said, all of these libraries have their strengths and weaknesses, much like any group of live musicians. Sometimes you need Steve Gadd; sometimes you want John Robinson for a recording date based on, primarily, the requirements of the music and what their "sound' is. Right? Same with sample libraries.
If you can only afford one library, then you will just have to investigate the options and decide on your own; no one of us could—or should—make that decision for you. These libraries are so deep that a simple comparative demo will NEVER properly represent each library at its full potential. All of these mega-libraries must be approached like new instruments; you have to learn how to coax the best from them and explore them in depth.
Regards,
Rob Shrock