As long as the car has been serviced regularly you will be ok, i generally service mine every 6k...
But you will be surprised how FEW people actually sit down and check the service history when buying a car.... so it isn`t as important as you might think..
i always like to look after a car, but, if the car your looking at has been serviced regularly it will be fine..
But back to your question... it IS whichever comes first like you first thought...
Scott