There is a lot of debate about the differences between Coaching and Mentoring, and which approach is better in what circumstances. Both Mentoring and Coaching are similar in that they are about personal learning and development, and the basic skills required by the Mentor or Coach are also similar, such as rapport building, listening and questioning, challenging and giving feedback. 

In practice, Mentoring tends to be more guiding and advisory than Coaching and in this regard, Mentoring is considered a slightly more directive (PUSH) style of intervention. Whilst a Mentor might Coach during the Mentoring process, a Coach will rarely Mentor during the coaching process, as Coaching is most effective as a non-directive (PULL) style of intervention.
So how do you decide whether you need a Mentor or a Coach? 
If you want to focus on the content of your day to day work and:

  • achieve specific measurable results in a time frame of less than 6/8 months
  • improve your work performance in a particular area
  • develop specific skills or adopt new, more effective behaviours to get better results
  • have a specific workplace problem, relationship or issue that is causing you to be less effective than you would be otherwise
  • change your attitude or mental frame to be more positive and effective in specific workplace situations or circumstances
  • be effective while going through a short-term transition such as organisational change, job relocation, promotion etc.
then Coaching is probably your best option. Expect to be working with them as your Coach for a period of 4-8 months, meeting every 3-4 weeks or so. 
If you want to focus on the broader context of your working life and:

  • learn more about the organisation you have just/recently joined (culture/systems/unwritten rules/politics etc)
  • focus on your longer term personal and professional development in the organisation
  • review your career options and choices over the next 12-18 months
  • seek promotion or have been promoted and want to ensure your style/approach etc meets expectations at this new level
  • re-invigorate your personal sense of purpose and motivation at work
then Mentoring is probably your best option. Expect to be working with them as your Mentor for a period of 12-18 months, meeting every 6-8 weeks or so.   
Whether you are choosing a Coach or a Mentor, you will be expected to be committed to the process and to make the most of the opportunity that Coaching or Mentoring presents, as your Coach/Mentor will be committed to your success.

In either case, you can expect to be challenged, stretched (in terms of your thinking and capability) and develop a deeper sense of self awareness and personal drive.