Hello Hoạt,
Firstly, let me congratulate on your getting this great opportunity from Google. You are the only Vietnamse - born, raised, and educated in Vietnam- I know who gets such an offer. :-)
As I've tweeted you, the best thing you should do, IMHO, is to reserve your study results at HUT and come to work for Google for 1 year (don't tell Google this though). If it fits you, then decide.
For eXo Platform, while it is a great start, I think the sheer scale of Google will open you to new horizons and possessing a good understanding of how the biggest technology firm operates will be of huge benefits to you in the future.
My mentor passed Google's 8 rounds of interview to become its Product Manager in SEA. It was a great opportunity but since Google is not doing anything great in Vietnam, he declined the offer a few months ago. However, Google Singapore is nice company with nice people, and this is something you should not miss.
Part of the reason he quit was because he had already had a great business in Vietnam, a great family to take care of. You're different. Right? :-)
Anyways, good luck!