Decision and PhD. Take a once-in-a-lifetime opportunity or pursue a life dream

I'm currently finishing what would be a Bachelor of Computer Science.

I have always always wanted to study in a top US university. That has been and still is my dream in life. In the past, I was in a program of the US embassy which trained a selected group of students (both in maths and English) to apply for an undergraduate scholarship in the US. I did not succeed at getting one (I only applied to above average universities and only to 3 of them) but that motivated me to strive at a local university so that I could make my postgraduate studies in the US.

I've been quite successful. I happen to have the best GPA of the best college for a Computer Science degree in my country. That being said, even though the university's level is great, it is definitely not a top university in the world (it is located in South America). I believe I really did put the effort. I studied about 12+ hours a day, 340 to 350 days a year, for 5 consecutive years.

Recently, I've been offered a PhD position at a good (though not a top) university in the UK. The position is fantastic, it is a full scholarship funded by Microsoft Research and it is definitely on a topic that I like (I was looking for a topic that would involve both programming and mathematics, and this position happens to involve Theoretical Computer Science, among others). This university is not a top university in the world, at least not on CS. My advisor did his PhD at the University of Oxford which is a top university in the world (top 5 according to some rankings). Recently, I've been recommended to him by one of his colleagues. Even though the university is not a top one, the advantages are that the opportunity is funded by the very Microsoft (which is a top company and can open some doors later) and that I will have a supervisor who did study at a top university in the world. This is the once-in-a-lifetime opportunity mentioned in the title; however, I never thought about studying in the UK before.

On the other hand, due to my good GPA and many teacher assistantships, I believe I am at a great position to get the Fulbright Scholarship (in my country it is not as competitive to get it as it is in others, and as far as I understand, there are 5 spots). I have a friend who has been awarded one and he led me to believe that I can get it. Also, due to the prestige of the Fulbright Scholarship, I believe that if I get it, I have great chances in getting accepted by a top US university. That would be to pursue the dream of my life, as the title says.

I can apply to (and plan to do so) both the Fulbright Scholarship and the PhD position funded by Microsoft. Both are due to June 2015 approximately, but if it happens that I get accepted by both (that would be unreal), turning down the PhD position will not be a good thing (as I understand, I would be commited to the PhD supervisor).

What I would like to ask is, as this is a place where many PhDs and faculty members contribute, what should I do? Should I take the Microsoft Research's opportunity (if it is funded by Microsoft, it has to be a top research program even though the university is not a top one, right?) or be patient and go for the Fulbright Scholarship? Also, how does a Microsoft Research scholarship compare to a Fulbright Scholarship? Please note that it will also be great for me if the PhD position later leads me to a great postdoc in a US university.

I'd like to note that I definitely didn't want to publish my accomplishments nor is it my intention to show off by any means. The only reason I spoke about them is to put everything in the table so that I could get an honest, realistic answer to the question.

Note: The Microsoft Research opportunity consists of a PhD position and starts about June 2015. If I get the Fulbright Scholarship, I would start a Masters Program in August 2016. Meanwhile, I would do a thesis work at my local university. I have no preference between a PhD and a Master; however, I understand they are two different things.

About what I am exactly seeking, I have to say that I really want to be the best academic/professional I can be (that's why I care about top universities), that I'm willing to put in the effort and that I felt very limited by the course levels and resources while studying at a local university which increased my will to study abroad (though I am extremely grateful for all that I learned and it's been good to me).

Please let me know if you need more details.