The NS3 synth also plays samples, so you should be able to recreate any sounds you like from the NP5 using the exact same samples -- you just have more options to tweak the sound also of that.
As for the action, the NP5 is more targeted towards piano playing. I really like the action, and it works well for me both playing acoustic and electric pianos -- among the boards I play regularly, it is the one that I like most for acoustics given that it is also very portable. I wouldn't want to play organ on it, though.
The NS3 HA88 has a somewhat different feel, and responds differently to your playing. For electric pianos, I really like it and it also works just fine for me playing acoustic pianos, although I prefer the NP5 for the latter part. For organ and synth parts, it works better than the NP5 (although organ is of course nicer to play on a waterfall board). So it's designed to be a jack-of-all-trades board -- and I've played it extensively since it was released and done a lot of acoustic parts on it also.
You may also consider the NS4, which is again different. I really like that board
But... we all tend to look for the "best" and "most optimal" feeling, and that can sometimes be a lifelong journey that never reaches a destination. I've learned to adapt to whatever I'm playing -- sure, if I jump from one board to another, I do feel the difference, but I also quickly adjust to it after a few minutes of playing.
So then it more becomes a matter of the overall ergonomics and features you need/enjoy to have for your playing.
And there clearly the NS3 (and even more so the NS4) are much more capable boards with a lot of features without really being much more difficult to use. And they can (also) bring you hours of joy playing
So do you need/want the additional sounds, effects and controls? Or do you want to focus 95% on your piano playing and excel with that?