The main difference is that while Synchron series products are recorded in a large scoring stage room, with plenty of that room in the sound, the Studio (formerly known as "Synchron-ised") series were recorded very dry indeed.
So it depends what you need: if you like to choose your own room sound, reverb, etc., e.g. if you use VSL's excellent MIR3D software, then the Studio series is the way to go. If, on the other hand, you prefer to have the sound of the room baked-into the samples then Synchron is for you. Both series are terrific. At present the Studio series also offer more instruments (particularly in the Special Woodwinds package, which has some gorgeous and less standard ones), whereas the Synchron series often have more different articulations.
That's the biggest difference, anyway. The other thing is that the Studio/Synchron-ised samples are from around 15 years ago, whereas the Synchron series are more recent. However make no mistake: both are superbly-recorded and very usable. Personally I have the Studio Woodwinds and Special Woodwinds, as I'm not after that big scoring stage sound, I prefer to choose my own reverberation, but that's just me. Both can be highly recommended.