Jean, remember that the headstone probably gives the date of death, but the PR is the Burial date not the date of death, they are not the same though normally it is within a few days and the PR may also give the date of death. The examples you give are unusual, particularly being buried before you died. There can of course be many reasons for errors, headstones can be erected many years after the event and wrong dates put on them, wrong dates can be put in the PR's and there can be transcription errors in both the PR's and MI's.
I have come across cases where people have been re-buried at a church several years after they died as they were originally buried at a different church but later moved 'home'. Burials several weeks after death may also not be unusual if the person died away from home and had to be transported back prior to the days of the railway.
I don't think you can give a definitive answer that one record is better than the other and like a lot of genealogy you have to make the best interpretation of the facts that you can. It can be worth going back to the original PR's just to make sure there are no transcription errors and if the stones are still there checking them to be sure they were not misread. sometimes there are photos on Billion Graves or other websites.