Px120
I thought the action on
the Casio CDP100 & PX110 were the same when comparing them back and forth. Really nice for the price, basically the best action you can find in that price range if your just talking about utilizing it for midi. Recently they came out with the PX120, which is replacing
the PX110, but I was only able to A/B the action of it with the CDP100. My first impression was that
the PX120's action is a little better, but not critically so. The CDP is going to be the cheapest of the three and my main hangup with it was that it only allowed for strait up sustain with one pedal jack. The others (PX series) you can expand to three pedals with the SP30 or SP30R (not sure what the difference is). Of course the on board sounds of the PX series are a little better too, but that really only matters when you want to practice a bit without turning everything else on or if you've taken your keyboard out of the studio for the week. Still it's nice to have that option, and they're all light enough (26lbs) that it's not a big deal to move them around and you'll have sound whenever you do that. I wouldn't upgrade from the casio till I'm ready for a higher end Yamaha, and even then if I did that, I'd lose a lot of versatility cause it's really heavy/clunky and without Speakers, not to mention it would cost 3-4 times as much. Maybe down the line, but I'm pretty content with the Casio's action.
Other options to look into could be like a Studiologic or CME 88key midi controller. Both are more expensive with no on board sound for practice sake. The Studiologic is heavy with good action, and the CME is feature rich + aftertouch. For my bucks the Casio's the best value for action & versatility, least in my applications which is to use it as a midi keyboard with virtual instruments. Pick up a cheap Midi controler like
the Behringer BCR2000 to use in conjunction with a CDP100/PX110/PX120 and your golden with the right VSTi's.
I think the only difference between the PX110 &
PX310 has got to do with on board sounds, memory and things of that nature. The Keybed is the same, or seemed so when I played them.