I think the only way to really tell is to listen to them side by side, since perceptions differ. However...
The CLP 170 has something Yamaha calls "active instrumental field control". Here's the marketing blurb:
Yamaha has developed the exclusive iAFC (Instrumental Active Field Control), offered on the CLP170. The iAFC outputs sound picked up by a microphone, and combined with the Clavinova's internal sound through rear speakers, makes automatic adjustments to create a richer sound that best matches the acoustic characteristics of the room you're playing in. This feature incorporates three separate effects: DDE (Dynamic Damper Effect) can recreate the breathtaking resonance that occurs when you press the damper pedal down on a grand piano; SEE (Spatial Ensemble Effect) reproduces the reverberations and resonance of a real stage performance for a much more spacious sound; and NSB (Natural Soundboard) can be used to pick up sound from the microphone to recreate a virtual soundboard — and assures an expansive, more natural sound.
The CVP 210 does not have iAFC (although it does have effects that sound similar, like string resonance.) Some people think it matters, others don't. Of course the iAFC effect per se only would be heard when you're using the speakers instead of headphones.
Another important parameter is the size of the piano sample. My best information is that the CLP 170 has a 36MB piano sample while the CVP 210 has a 112MB sample. That is likely to make a big difference in sound quality - bigger samples have more room for multiple keystrike levels, need fewer loops, sample more keys, etc. On my CVP900 I use a separate computer-based sound source that has gigabyte-sized piano samples and you can really hear the difference. But again, ya just sorta gotta listen to both if you can.
Finally, the CLP170 has the "GH3" keyboard as does the 210. The CLP175 and CVP 900 (mini-grand case) have the "natural" keyboard with wooden keys and a totally different mechanism. I believe the natural keyboard has a markedly better feel - YMMV.