Candour means the quality of saying what you think openly and honestly.
Diplomacy is a skill in dealing with people in difficult situations without upsetting or offending them.
I am not against anyone of them but I feel being candid is better than being diplomatic. When you are candid, you do things frankly without sugarcoating your words but when you are diplomatic, you tend to tiptoe around the truth more often. You use your tact while talking, your heart does the processing instead of your mind. Although, I do not like tactless people but in applying tactics, you should be candid.
You should be of nonconformity than of sensitivity. You should be frank because when you tiptoe around the truth, you tend to tell lies too.
Before you approach any issue, you should realise that your diplomacy should be juxtaposed with your frankness so you wouldn’t end up hurting anyone and you wouldn’t tell lies.