I can say that with all default settings, the Synthogy American Concert D has a brighter, more silvery sound compared to the Alicia's Keys plugin. I find it produces a better sound for classical music, while Alicia's is better for contemporary stuff. Also, the Synthogy plugin is better for faster paced music, while Alicia's Keys is better for slower, mellow music.
Up to 1,000 voices (polyphony) can be used (I recommend using an SSD to store it for this).
The only thing I don't like is the iLok thing. It's $40-50, and it was annoying to set up. It works, but it's annoying. Also, for some reason whenever you restart the computer, you have to unplug the iLok and plug it back in otherwise the computer won't detect it. I don't have to do this with any of my other 7 USB devices. I'm also concerned about the fact that if I lose this little thing, not only would I have to buy another iLok to use the plugin, I'd most likely have to purchase another LICENSE too!