If given the option to apply for one of two positions at a company, one that I am definitely qualified for, and one that I may be slightly under-qualified for, which should I apply for and why?

which ever big one - senior - you apply .. and  make / change / modify your covering letter accordingly. Please keep in your mind one thing 'no human being is 100% qualified to any position - whatever your education . We merely feel. assume act as if we are qualified. So, there is no qualify or no qualify but the question is how you can market yourself.  So better aim for moon - you may end up at stars.

Apply for the SMB position and let them know you are interested in the enterprise position. When the enterprise position becomes available in a year or two, you will be the first person considered.

In general, if the company has a solid personnel (translation: training and development) infrastructure, then I would personally aim for the position I am slightly under-qualified for. I'd be thus hired on potential with the understanding that given appropriate training and development, I would perform within or beyond expectations at the position. That said: It may get more complicated than "in general" -- it often does. Some thinking points for you: Who will be your boss and how does this person manage? What is the track record of direct reports under this person? Have they gotten promoted or do they seem to move out of their positions into other lateral positions or other companies? A boss that has a mentoring mindset can help unleash your potential and even help you discover potential you didn't know you had. A boss that does not have a mentoring mindset would prefer you to "be ready to show results, now." Just because a company is publicly traded does not mean it devotes much care to employee training and development. I continue to be surprised at how often employees are hired with an implicit understanding that they should "figure out things on their own." The more specialized a position, the more prevalent this implicit expectation. I'd look into how much this company cares about employee development. Account executives and client-facing roles tend to have solid (sales-based) training programs but often at the entry level. The higher you go, the more customized your skills development needs to be, and I'm not sure a lot of companies do a good job "training" at the higher levels because it is harder to develop "standardized" developmental tools across the rank. Hence, my prior point about the boss. Not all factors of success have to do with you and how ambitious you are, unless your ambition is matched by your unparalleled resourcefulness. You can have pages of goals and ample ambition but if you don't have a track record of getting resources for yourself when it is not already provided visibly within an organization, you may have a steeper learning curve going into a new company slightly under-qualified for the position you've won.