Is it a good idea to ask an assistant professor to be your supervisor?

[Soe Nyein Oo]

I am currently sitting for MEXT Japan Scholarship test for master degree. I have already had an acceptance letter from one university and just decided to try for another university as a safe option. There, I found one faculty member whose research fields match my research plan but he is an assistant professor. So, my question is that is it really a good idea to ask for an assistant professor who got his doctorate four years ago to be the supervisor. Yes, it has to be that university and that one professor is the only one whose fields match mine.

I hope for some kind replies pretty soon. Thank you!

[fuzzylogician]

In general I would advise against going to a school where there is only one researcher with interests similar to yours. Regardless of tenure, what if that person gets sick or leaves, or you two just don't get along (which does happen!)? Even if you do get along and everything is wonderful, you will need to be able to put together a committee for your dissertation/thesis, and you will need to have at least three people who can write you LORs if you want to apply to any job or further education. Having just one person is not enough. 

As for the question of having an assistant professor as your advisor, there are good and bad sides to it. There is always some risk that s/he will not get tenure and will have to leave before you graduate, and then you are stuck. There is also an option that they are a rising star and get poached by a better school. Assistant professors will often have a better understanding of what it's like to be a student and to be on the job market, because they have had those experiences more recently, and they may be more driven to publish, because they are on the tenure track. They may be more easy to relate to and (as a sweeping and not exceptionless generalization) they may be more supportive of diverse applicants than some older professors. On the other hand, they will be less experienced and may take time to adjust to advising, and they may be more demanding than you'd like in terms of work hours -- so there are potential good and bad aspects of each option. At the end of the day all that matters is what the particular person you are considering is like. You should talk to them and to their current students to get a sense of whether you'd get along with this person.