I have such a dilemma right now. I've just tested P525 and P515 side by side and to my surprise, I slightly prefer NWX over GT-S 😮 It's easier to play softly on the former, maybe after some time I would adjust to lighter GrandTouch-S, but, well. Not what I was expecting. I've spent around 20min on each.
I really like sound of both via internal speakers, the clarity of CFX sample really shines in that scenario. No difference in that aspect between the two. Played a few chords on ES920 standing next to Yamahas and while I prefer Yamaha sample, Kawai speakers, while not as 'loud', might be better quality-wise. Headphones - well, I'm so used to Garritan, VSL and Production Voices CFX that P5x5 sounded 'meh', so with headphones I will use VIs 100% of time.
Unfortunately there was no FP90X in the shop, so I played some LX (705 or 706, not sure) with PHA50 and PureAcoustic engine. I liked that action the most. So easy to control dynamics, both ppp and pound it hard to fortissimo. While the sound might not be as nice as Yamaha or Kawai, I think via internal speakers it might be sufficient.
Eh, I would like Roland action and features with Yamaha sound and Kawai speakers. It would be perfect. For now, it's P515 vs FP90X for me.