I have tried both these models and I want to offer a brief description of them. Between the two models, there isn't much difference other than the 71 offers 192 note polyphony and the 51 offers 96 notes, which for most piano playing is fine, but bear in mind that the pedal requires some of this polyphony.
KEYBOARD RESPONSE & SOUND QUALITY
The thing that strikes me about the piano is the mechanism. It has nice heavy wooden keys, on both the whites and the blacks. The piano tone is pleasing and the other sounds are quite realistic, particularly the harpsichord.
I think that the speaker system lets it down but this is the case with all digital pianos - with the possible exception of the Yamaha 380!
Although the keys are heaver I think that the Yamaha has the edge over the Kawai in terms of response and clarity of tone. The Kawai is perhaps just a little too dark sounding, which doesn't translate well onto a digital instrument.
One really nice thing about these pianos is that the music desk is the same height as it is on a grand piano, so ergonomically speaking, there is no difference between the acoustic and digital instruments. The cabinets are of high quality and look like proper furniture.
They are slightly cheaper than the Yamaha but don't offer as many features. The CA71 was about 2500 and the 51 was 2000. If money wasn't the issue, I'd be swayed to buy a CLP-380 over either of them, however it is a nice alternative at these prices.