Only you can answer this question, because only you know what you want out of your Masters & what your risk appetite is. Superficially, the choice is fairly straightforward:
If you take the offer:
- You're paid $15,000 for however long the Masters is, not to mention a tuition waiver. That's pretty substantial for a Masters student.
- You are further guaranteed a long-term research position. This is a big deal since the post-graduation job search is a major headache for many students.
- You are presumably in a familiar environment. You know the local city. If you have a significant other or children, you have less trouble accommodating them.
- Your experience in the academic aspects of the Masters might be better simply because you know the supervisor personally.
- Flip side: you could suffer from the "same university" drawback.
If you look abroad:
- A more prestigious university is going to affect your job search, especially if you get a non-academic job. If you stay in academia, then the reputation of the department matters more.
- If you get along well with your advisor, there's a good chance your experience is better at the more prestigious university, for reasons such as more good students to talk to, more seminars by leading academics to attend, etc.
- Warning: you might not get admitted.
- You have a lot more administrative hassle to deal with, e.g. you might have standardized tests to take, visa applications to worry about, housing to look for, etc.
Ultimately, is a bird in hand worth more than two in the bush for you? If you say yes, you presumably dislike risk, in which case it's better to take up the offer. If you say no, the reverse probably applies.
One thing worth pointing out is that you can still become an academic if you stay at the local, unknown university (in fact, the long-term job offer is effectively a guarantee of this). However, because of the Law of Descending Prestige, you probably won't end up at a well-known institution. It's still a bird in hand vs. two in the bush situation, and it's up to you to choose how much risk you're willing to take in pursuit of your dreams.