I have had several mentors over the course of my professional career. Some formal, some informal, but in all cases, their input was valuable and appreciated. Having a good mentor is like a gift; you have someone who cares about you and your success. Someone who will listen to all sides of the story and help you determine the best course of action. And best of all, a mentor will not benefit from your actions/decisions. Their win is watching you succeed. So whether you take the promotion, sign the contract, hire the terrific new graduate or not, they are the partial party to help you see all the angles.
Some people have a mentor at the office and some have a mentor outside the office. Both are valuable. A mentor within the office knows the detail of the office dynamics…the players, the politics. They can be helpful when looking for guidance within the company. A mentor outside the office is beneficial when you want someone, who, regardless of company or position you have, is there for you to discuss your career, progress, options, etc.
Whether inside or outside the office, if you are looking for ongoing career development, help and feedback from a neutral party, find a mentor. Find someone who has been where you are trying to go and who has a great track record. It will be well worth your while.