When considering student loan debt a big question that needs to be answered is your income potential after you graduate. Will your income be boosted by a master's degree? The answer to that question should weigh into your decision. Additionally you have to weigh the possibility of an employer paying all or part of your graduate studies.
If you were some kind of engineering student, I would advise not to take the fellowship. First you won't start off that much higher salary with a Masters, and second most employers offer tuition reimbursement so you would have a real salary and have a good portion of your school covered anyway.
If you only have 15K in student loans you are not in terrible shape as long as you are obtaining a marketable skill. Once you get out of school, still live like a student until the loans are paid.
Even if you have a degree program with something like psychology you might be better off going to work. From what I understand such a degree pays almost nothing for a BS, but a lot more for an MS. You could do like my son is doing and go to work in the police department. With overtime it is possible to earn like 90K, and the department pays for his schooling. That is much more attractive then the kind of fellowship you are talking about.