I do not agree that the Synchron Series is a step forward. It's just different.
From my perspective, VI-series is what makes VSL unique, still to this day. Ever since the very beginning there have always been libraries that were recorded in some remarkable scoring stages and then marketed as "The Ultimate Film Scoring Library!". For many, many years VSL never fell into that trap and instead proved the VI-series to be the most detailed, versatile and flexible virtual instrument out there. It was not only for those with only film score in mind but also for everyone interested in orchestral music of different genres (not least, classical music).
I own them both and have different use cases for them, sometimes I mix them with great results.
However, if I had to pick I would always go with VI-series. I could replace the Synchron Series with other vendor products, but I could never replace the VI-superheros - there is just nothing like it.