The NP31/NP11 have a nice feel for a keybed that is "unweighted", much more playable than similar offerings from others, in my opinion. I think the same 10-voice sample sets are used in these as in the P85/P95 instruments.
The P85 and P95 do not have the same 10 sounds as each other, two are different.
If the NP31 has the identical piano sound as the NP30, it is very similar to the P95, but not quite identical. If nothing else, there is different note stretching going on at the bottom of the board. (Possibly related to the fact that the NP30 has only 76 keys.)
A disappointment with the SP-170 was that only loudness changed with different velocities. Go for the NP 31.
When I tried an SP-170, I felt it did have at least one velocity switch, that occurred at a relatively quiet level... in fact, it was one of the things I did not like about it, because if I was playing a quiet piece with a bunch of notes right around that velocity break, I'd hear very noticeable changes in the sound among the notes where my finger pressure had hardly changed at all! I have a recorded sample of that I'll get around to posting one of these days...