Which is better: to achieve my master degree in usual time or use extra time to prepare myself for a PhD?
I suppose it might vary between different countries, and I have no idea how things work in Brazil. Here in Europe, extra classes would probably have some value, while the fact that you finished earlier - not so much. If you could start some actual research activity it would also be quite helpful in your application.
Another aspect is that when applying for a PhD you might need reference letters from established academics, and maybe you could use this extra time to impress someone and make sure the letter you'll get will be stronger.
Of course, all this neglect the financial aspect of this decision, but since you mention you have a scholarship, maybe this won't be a problem for you.