Note: I could not find CDP-220... Only AP-220, polyphony of 124. Perhaps you were refering to Casio Celviano AP-220 instead?
64 is more than enough, even for complicated pieces. You won't hear any drop out of notes, unless you do a lot of track recording on the digital piano, where the problem is more apparent with polyphony 32 with tracks overlapping, playing many notes simutaneously, e.g. bass, drums, synths, percussions, loops, etc... Which you won't be doing on a digital piano. However, you may be thankful for choosing the one with 128 polyphony. The higher the maximum polyphony note, the higher the sound quality, in one aspect. On the other hand, looking at both the specs for AP-220 and YDP-140, both units are only price comparable. The AP-220 seems to be a better unit than the YDP-140, in terms of specs, i.e. AP-220 has a slightly more powerful amplifier, 10 more piano voices, additional recording track and double the maximum polyphony compared to YDP-140.
I'm not particularly impressed with the hammer action of Casio digital pianos in general. I've tried several at showrooms and I have to play on one every week for classes but maybe I'm not used to it. It's a very subjective matter. I also lean more towards Yamaha's piano voices. They sound more natural. Do try both units if you can find them at showrooms near where you are.
Just to add, looking at the specs for YDP-140 and comparing that to YDP-160, it seems that the YDP-160 unit has a better graded hammer system (GHE) than YDP-140's GHS. YDP-160's amplifier is much more powerful too, therefore if I were to try it side by side, I'd expect the YDP-160 to have a better quality sound than the YDP-140. So essentially, the extra 400-600 bucks is for a better sounding unit with improved key action.
I currently own a Yamaha Clavinova CLP 330, GH3, 20W amplifier, 13.5cmx2 speakers, 128 polyphony, 3 songs, 2 tracks recording, 14 voices. Good sturdy unit, reasonable price. I'm thoroughly happy with it. Will definitely look into an upgrade to either a higer end CLP or CVP series in the future.
I hope this helps. Good luck!