Most acoustic VSTi's are designed around extensive flexibility and multiple options for the SA abilities. On the whole, they tend to get designed for studio use, where you can edit the control parameters after you have played the basic part. Hence, they can be made to sound (with work) quite fantastic. And STILL sound pretty good as you play them live.
But Yamaha's SA voices are designed with live play as their first and ONLY priority. Compared to a VSTi, they don't offer as many possibilities and articulations as the best VSTi's, but they are designed to be played LIVE, and very carefully crafted to be best at that ONE task...
If you are an editing guru, it's quite possible to massage the VSTi to be better than the SA sound for live use, but it's rare that they come out of the box as superbly designed for live play as Yamaha's voices are...
On the other hand, the CHOICE and range of VSTi articulated sounds is FAR beyond what Yamaha offer. It all depends on just what you are looking to do, IMO.
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An arranger is just a tool. What matters is what you build with it..!