Compared to the other FF-Prog I know, I compare both as follows:

KLM FD Service-Points: Status Miles. Normaly you need 15K Service Points to get SilverWing. If you are Silverwing, you can get Royalwing after another 24K Service Points.
They don't expire, but if you get Silverwing they take away 15K Service-Miles off your account and for Royalwing 24K. This happens every year. So they don't expire but be taken away for Elite level (and requalification).

Bonus-Points: Bonus Mileage. This is the same amount as for Service-Points but additional Promos (e.g. double) or Points earned not by flying KLM/Wings can come into your Bonus-Points account. Those are good for rewards/upgrades.