If I choose for dynamic, I have to check the dyndns.org website at least every month. Or else the hostname and my account get deactivated. My IP is dynamic, but it is possible to have the same IP for a whole year. It (most of the times) only changes when my ISP is working on their systems.
It would be nice if dyndns let's me forward the hostname to the hostname given to me by my ISP. Because that's the only thing which is static. But as far as I know dyndns doesn't let me.