I also prefer ViPro. But I wish they would update the gui a bit for 2020, HiDPI support, among a few other things.
I chose the Cube universe because its just an ultimately more complete and more powerful system with MirPro, many rooms to choose from, with the Cube FULL, appasionata, dimension strings and brass and basically most other silent stage libraries...its a very complete set of instruments and articulations, with MirPro capability to setup the stage and room however you want. For true power users its way beyond Synchron. Its not going away. I'm not complaining about it lacking anything either, I have barely scxatched the surface of what it can do already right now. I hope the VIPro instrument will get a little more update and love, things like bigger scalable fonts, HiDPI support, etc.. I personally think the matrix concept, the humanization features, etc..its a very powerful instrument.
I think Synchron player perhaps has some wider appeal because its easier to turn it on and just start working with a much less intimidating GUI, built in room, etc.. its more plug and play. I've been picking up some of the syncrohized stuff on sale and plan to work with it more and more to see how I like it, though I also like using MirPro and other rooms, so really for me the synchronized stuff is more about a simpler GUI then it is about the built in room. But for a lot of peopel new to VSL, synchron will havce a more affordable price point of entry with the built in room and decent sound without having to invest in MIR stuff or get deep into the technology for that. I think if you look at most other libraries out there, they tend to be designed more like synchron is, with built in rooms and simple interface. While VIPro has a rather intimidating GUI to begin with, on first look you take in the matrix and think "how should I use that?", etc. But ViPro is also very impressive to me as a symphonic instrument plugin..I think its best in class in terms of articulation control, humanization, ability to change attack points and times and really stretch the existing library in many ways...I thihk most people barely scratch the surface of what ViPro can do, including me. That's why synchron may ulitmately be more useful for the vast majority of people. But at least for now I ahve over $10k invested in ViPro silent stage + MirPro...and I'm committed to that platform, so I hope VSL will continue devceloping it...including enhancing the ViPro instrument itself.