lookup vs optionset

Ok folks I am just trying to get a clear understanding or a general rule of thumb

Lookup vs optionset 

when is it best to use either one

my understanding is along with relationship it is best  to use lookup if the lookup values change   and optionset if values never change   and I understand if you remove values from either one you may ruin the link to the record

so clearly I am not sure in which type of situation to use either a optionset or lookup 

can someone give me some golden nuggets to remember for future here :)

Thanks