Are they really same age and condition? Has your trusted piano technician given you that assessment or is it something that the owners (or store) told? Never buy a used piano with [**] an inspection by a technician, it's really minimal cost (about $50-$100 depending on location) and invaluable for avoiding the risk of getting a dud.
If they are **really** the same, get the one you like most. If you like them equally, get the cheaper. Note that people tend to like Yamaha more than Kawai (I don't) so that usually commands higher prices.
[**] EDIT obviously I mean WITHOUT!