I have a rule of thumb that if it costs no more than twice as much to buy a score as to hire it, I’ll buy. (This seems to happen increasingly often.) There are advantages to singing from a score you own; you have exclusive control over all markings on it, and can leave in your own personal ones relating to particular difficulties you have. And you won’t have to pay in future to hire it.
But there is some pleasure in using a score that many others have sung from before you. You can for example boggle at some of the peculiar interpretations which your predecessors had to follow, or observe how you are not the first to find a certain passage tricky. At one recent concert our scores of a rarely-performed work were, I estimate, about 60 years old and had accumulated a fair amount of annotation. (As well as using some old-fashioned typography: crotchet rests a mirror image of quaver ones, repeated notes in the accompaniment indicated by slashes, and dots separated from the notes they lengthened. I don’t think I’d ever encountered that last one before.) My score included details of someone’s future travels (to France and New Zealand) on the flyleaf and some instructions (in a different hand) in Welsh!