A mentor is someone who has done what you aspire to do. Hence, he has a personal, direct experience with what you want to accomplish and can guide you accordingly. If you want to start a business, you want a mentor who is successful in your desired business. If you want to become a scientist, you need mentoring from a professor, accomplished in your field of science. If you want to become a Buddha, study with a spiritually realized guru.
Mentoring is more aggressive, pushy than coaching, which usually lets you figure out what you need to do on your own. Mentoring is more real, requires more commitment from you, and therefore, is more effective — you will attain success faster. Mentoring also involves far more trust in the mentor. This trust develops gradually: so, it is a good idea to start with a coaching relationship and then let it naturally progress toward mentoring.
For things spiritual, finding an able teacher — who is willing to take you on as a student — is the only shortcut on the path that accommodates no shortcuts.