All subjective.One can say,

Superior
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Bk9 is superior for light weight /76 keys/maybe drawbars /org sounds and good drums piano sound/CS

But inferior for lack of dynamics in style writing (nothing to do with dynamic arranger),no after touch for more than $2000 KB ,bad display, 3 way only joy stick,no multi pads ( I know audio key) etc

PA 900 has superior for detail and deep sound/seq editing (yes more than Yammy and Roland),large touch screen, good punchy sound,4 way joystick/song book/sample reader/CS

But inferior for lack of style support (korg owners are always bagging for conversions 3 months after they bought the KB and found out that busy styles don't really work in the real world and muting is not the answer because busy patterns/bass lines are the problem ,good when you play as solo but problems start when you start to sing on those styles /most will only use drum pattern and use piano/strings and sing on top of it
/too synthy and not so great acoustic sounds for some


S750 is superior for SA sounds guitars acoustic sounds /reed /woodwinds (yes both Roland and Korg 's TOTL models can't touch it ( they still have a long way to go) and mega style support (much more than all other manufactures combined) including simple non busy styles perfect for vocals,easy song creation since most 3rd party styles are avail for free,true backward style compatibility (try that on Korg)
Same hardware and efx like upper model (950) minus VH and other obvious differences ofcourse
Remember , there are already converted styles from most arrangers (korg/Roland/ketron/technics) for Yamaha,not the other way around (Due to hard to convert Mega voiced style format of Yamaha)/ SMF with style drum play


Inferior in hardware/bad quality on mod /pitch wheels (even $500 MX49 has better wheels),not so great key feel ,less punchy drums/no CS/