The demos of Hollywood Strings are a turnoff instantly to me. The sound is not as good as the Appassionata right up front.
But also the interface of Vienna Ensemble has become essential to me. I hate having to use something else that is inevitably less elegant.
And the fact that all the VSL strings in general are not one library, but a whole combination of libraries that are diffferent and have been amassed over a long time - the Solo Strings, Chamber Strings, Orchestral Strings, Appassionata Strings - it is a huge variety of sound you don't get elsewhere. And Dimension Strings are probably the next step...