I will try to describe my situation briefly. The university I'm currently studying at offers a 4-year Bachelor program in mathematics.
I am currently in my 3rd year and the program for the 4th year consists only of optional courses (which I choose). As I have taken a rather high amount of optional courses so far, I can satisfy my ECTS requirements for the whole program in 3 years. The university, in this situation, would allow me to graduate at the end of my third year. Then, I could pursue an MSc. degree at the same university for a year and graduate with an MSc at the end of my 4th year of study in the university.
I would like to note that this has been done before by other students here and I'm not fantasizing.
My intention is to apply for a PhD at a university in the USA after the 4th year. (I am not a US citizen and I haven't studied there).
My question is: would getting an earlier BSc and MSc degree affect (negatively or positively) my chances for admission in a top university in the USA? Should I just graduate with a BSc. in math? I know that sometimes having a MSc. from the same university can be a negative sign. Should I be worried about that?
Some other background: My current university is not really "elite" of any sort; it barely enters the top 600 in the world. I have a few published articles and I have participated in conferences. I am also currently pursuing an internship in a research institute in the country.