Technology-wise the better bike is the R6, hands down. Motorwise, R6. Chassis wise, I’d say R6 too although Buell has some interesting ways of implementing things, and Buell does make good chassis that handle well.
Probably comes down to cost and the motor and what kind of sound you like to be honest. Yamaha’s motor is tested, tried, and proven throughout the years. Rotax, :gotme: . If you are a current or previous R6 owner, then you already know what you should be getting.