Are you talking new or second-hand or "reconditioned"?
Both are potentially
very good pianos but equally, there are abused and worn out instruments about on the second hand market.
It comes down to trying each and decided which has the best feel and tone. The Kawai is generally considered to be more mellow than the Yamaha. Note that both are normally quite powerful instruments and need to be in a reasonable sized room. Of course they can both be voiced, but that is not without problems and its best to go for the right sound in the first place. There are several previous theads on Viva Piano about Kawaii vs Yamaha, which are well worth searching out and reading through.
When I had short-listed my chosen pianos last year, I spent between four and eight hours playing each in the respective shops before I made my final choice. I was at one big shop in Leeds for three whole days (and still feel slightly guilty that I did not buy one from them in the end). Interestingly, the piano I actually bought was one I played for least time (about 3-4 hours). I just knew it was the right piano within seconds of first touching the keys ? I just couldn?t stop playing it!