Having played a number of one-piece Yanagisawa sopranos (older and newer) and a few newer two-piece designs including the S-991 and S-992, I prefer the one-piece design as I get a darker, less resistant tone. I won't theorize why that is - it is simply my observation. If you prefer a curved-neck option for comfort's sake, then that makes the choice pretty clear. Also, I'm a value-oriented shopper so I would opt to go with the used S991 if I didn't have a preference otherwise - but that's just me.