I've never played any of those three, and I'm a little more familiar with Yamaha keyboards than Casios, so I'll just make a few comments about the NP-31 versus the YPG-235.
Voices, or instrument sounds-- The NP-31 has only 10 different voices to choose from, all of them basic keyboard instruments (piano, organ, harpsichord, etc.) except for one strings voice and one vibraphone voice. The YPG-235 has close to 500 different voices, including sound effects and drum kits.
Layering and splitting-- The NP-31 lets you combine (layer) two voices together, and you can separately shift each layer up or down 1 octave, but you can't split the keyboard. The YPG-235 also lets you combine (layer) two voices together, and you can separately shift each layer up or down 2 octaves, plus you can split the keyboard (select and play a different sound for the left-hand side) and shift the split voice up or down 2 octaves.
Styles, or auto-accompaniments-- The NP-31 doesn't have any built-in styles or style-playing capabilities, whereas the YPG-235 does. Again, you might not care about whether the keyboard can play an accompaniment for you.
Registrations-- The NP-31 doesn't let you save and recall any keyboard setups. The YPG-235 lets you save and recall up to 16 different keyboard setups-- i.e., the selected voices and their various settings, the accompaniment style and its tempo, the different types of effects (reverb, chorus, and harmony), etc.
Recording-- The NP-31 doesn't have an onboard MIDI recorder for recording and playing back your own songs. The YPG-235 has an onboard MIDI recorder so you can record and play back up to 5 songs, with up to 6 tracks per song (5 voice tracks plus 1 accompaniment track).
Bottom line-- Try to play them yourself, preferably side-by-side, so you can hear and feel how they compare with each other.
