After waiting for some months, I finally got positive replies from the US and Japan.
For the USA, I had a position in UCLA or CalTech and a Fulbright scholarship offered. For Japan, the same in Tokyo Institute of Technology with a Monbukagakusho scholarship (Japanese Government scholarship for foreign students).
Just after arriving from Egypt, when I came back to ESA, I had these two confirmations waiting for me in my mailbox... and a very strong deadline: for the Japanese scholarship, I had to reply before 4 pm today!! That left me with only one day to take such an important decision.
I have been thinking without rest for the last 24 hours and I have finally decided to go to Japan. It has been an extremely hard decision, but I think this posibility can bring me the personal stability I need to work towards the PhD degree and it is very good from a professional point of view.
What can I say about Japan? After my first experience there (3 months in 2006) and two visits to my girlfriend, I really like the country and that is very good to help me go faster towards the completion of the PhD program.
Time will tell if this was the right decision. At least I know one thing, this looks to me the best current possibility under the current circumstances!
Therefore, there I go back Japan!!!