I would go for VI Pro, because there are some features I'm missing in the Synchron Player. I hope that these will come in a future update.
- Stretch function: I don't use it often, but you can achieve variations.
- Auto-Voicing: It's great for the Dimension series, but I would prefer an Auto-Divisi function (similar to LASS).
- CC thresholds: The new controller curves in the Synchron Player are good, but I prefer fixed values (as in VI Pro) instead of percentages, e.g. faster legato at a value of 80. Actually, I like Synchron Player, because you can define the starting and the ending position of a CC. @VSL: Please make this available in VI Pro!!!
- Interval map: This little feature is so important for me. With this I can imitate Spitfire's performance legato, because I can have a staccato/spiccato/marcato attack ONLY on the first note of a legato phrase. With the Synchron Player marcato presets you have the attack on EVERY single note. @VSL: Please make it available in the Synchron Player!!!
You can bypass the ambience in the Synchron Player and use the SYNCHRON-ized version in MIR Pro instead. The impulse responses are based on the MIR technology.
For me, one of the most positive thing is that the SYNCHRON-ized libraries are less expensive and you get the full library, e.g. Dimension Strings I.