The CLP170 is already inferior to the current CLP270 in terms of keyoard response and piano sound quaility. The real contest will be between the CLP280 and the CLP380. Keyboard response - as far as I know so far - is identical if not very similar at the least, but where the CLP280 has 3 layer dynamic sampling, the CLP380 will have 5 layers of dynamic sampling. Basically, it will be more expressive and realistic to the ear.
No doubt, the CLP380 will be 'better' and will push the limits on consumer cost after its first release, so it leaves you to decide which is the best model for what you can afford. A quick quote online is showing prices above 3000GBP, with the polyester model almost reaching 3500GBP!