Reference letter from visiting faculty vs unrelated faculty? We have a visiting faculty in our department, her main research area encompassing Embedded Systems, which happens to be my area of research as well. I have worked with her on my Minor Project (Undergraduate). I asked her if she could write a recommendation letter for me, and she said she sure can but I should approach other Professors/Assistant Professors who are not a guest faculty in my department. (She doesn't yet have that "Prof." in front of her name.)

There is one more Professor, with whom I haven't worked but have taken four-five courses and hence he knows a lot about me and my abilities. However, since I am applying for MS, prior research experience of course matters. Also, the problem is that that Professor's field doesn't include Embedded Systems.

And since my research field is going to be related to Embedded Systems, wouldn't it be better to get it from the teacher who teaches Embedded Systems and with whom I have worked?

I am confused as to which faculty to get the reference letter from. Would it be beneficial to get it from the teacher I have worked with but is a visiting teacher?
If you are applying to a M.Sc. position in a particular research area, and there is a senior researcher (i.e., someone with their PhD) in this area who has supervised your work on this topic and can speak positively to your research abilities, then I think it would definitely be beneficial to get a letter of recommendation from this person, whether they are a Professor yet or not.  At the same time, a professor with whom you have taken a number of courses could also provide a strong reference for you.  And yes, if they are a more senior figure, then that may carry more weight with an admissions committee.  A senior person may also be better able to compare you to other students that they have encountered over the years, and, if this comparison is positive, this can be really compelling in a letter.  

But there are a number of other factors you need to consider in getting the best possible letter of recommendation.  First of all, the letter needs to be positive.  Secondly, it needs to contain specifics.  Third, it needs to speak to the requirements/criteria of the position/scholarship that you are applying for.  So you need to figure out which of these people can write the better letter, all considered.  If you were to provide them with some talking points for their letters, which set of talking points would be stronger?  

I am surprised more students do not simply ask the person whether they can write a positive letter in advance.  It's amazing, but you do occasionally see support letters that actually criticize the applicant.  I think it's fair to establish in advance whether the referee can indeed produce something positive.

Of course, cultivating both of these relationships would be a good idea to help you in your future career.  There may be other cases where you need three letters (e.g., for scholarship applications).  Some people also list references on their C.V. in a special section.  You might ask them whether would mind being listed in this way, whether you use their letter or not.  When an applications committee reviews your application, they put a lot of weight into references from people they are familiar with.