I have a Motif XF and have grown quite fond of the piano provided with the Motif XF. It is very full, bright, and blends in quite nicely with orchestral pieces. This is a playlist of half a dozen or so pieces I have written, recorded directly off the board.
When I listen to both the Bosendorfer and the Vienna Imperial, they both sound flat to me. I am aware that these might possibly sound better with some EQ and light compression, however, since I am not a sound engineer, this is a daunting task to recreate the same type of sound.
When I listen to the Yamaha FX on the other hand, it is much closer to the same type of sound on my Motif, with exception to the mids being a bit high for my taste.
I am not familar with the Synchron series (to give you an idea, I am working with the Special Edition library as I am a full time student on a tight budget) with a couple MirX venues and Instruments Pro.
Anyway, to my ears, the Yamaha sounds much better, and as best I can tell, the standard library would service my needs fine, and, it is half the price of the Vienna Imperial.
Strongly considering buying the Yamaha, when money allows for it. I really think the Vienna Imperial pricing is a bit high, especially considering, in my opinion, the Spitfire Hans Zimmer Piano demos I have listened to have much more nuance to them, they sound much more natural, especially with more softer passages, and it costs less. The Yamaha Synchron also sounds quite good, I just wish there was more to listen to with a comparison between different dynamics.
Anyway, those are my thoughts. As a pianist first, composer second, my piano has to be spot on. Im super picky about my piano sounding and feeling 'right'.
Oh, and just fyi, the piano included in the special edition? Absolutely awful. There are free piano sound fonts that sound better than the one provided with the Special Edition (might be worth looking at what is provided with the Special Edition and bumping the 'quality': velocity layers, more samples, whatever it is that makes a soundfont sound more natural).
My recommendation, if you are looking at which to buy, I think the Yamaha Syncrhon is the way to go.