Authentic...?!!

Sorry, but do you mean the guitar voices you PLAY, or the guitar voices that the arranger plays? And, do you mean acoustic or electric?

Not that it matters a whole lot... IMO, the Yamaha's have possibly the most authentic sounding guitars in any arranger at the moment, particularly the guitars played by the arranger. Those Mega guitars, though close to impossible to play well from the keyboard, work SO well in styles.

I've got some problems with the S900 overall (keybed and drum sound, primarily), but I'll be the first to admit the guitars pretty much rule!